What is purple prose? Well, I could expound upon the intricacies of the particulars through a contextualised example, but there is the impending prospect that you, the receiving party, will be lost in the entangled forest of literature.
Now, if you you somehow managed to get through that last sentence and you're now thinking to yourself; "well, that was needlessly wordy", then congratulations, you know what purple prose is, even if you didn't know how to describe it before. And you may have gathered that purple prose is no good thing, even if you can't quite pin down what it is.
Purple prose in creative writing is what Mariah Carey does when she decides to sing entire scales in the middle of a tune, or when a dish is swamped with flavourings to the point that it ruins what would be a perfectly good meal. Purple prose is the use of writing so ornate and ostentatious that, rather than enhancing the narrative, detracts from it and draws attention to itself instead.
Now, is this saying that you should aim to never put any kind of decorative language into your work? Absolutely not: like the two examples above, everyone still admires a good singing voice or a careful sprinkling of herbs and spices, just as your fiction will benefit from the occasional flourish. But that's just the thing: the line between clever and colourful writing and flowery melodrama is fine, and not everybody agrees which is which. So let's look at three ways of saying the same thing with varying degrees of purple-ness:
SIMPLE: He stood up.
INTERESTING: He launched from the chair as though electocuted.
PURPLE: He propelled himself out of the old rocking chair so fast it groaned across the wooden floorboards, and a lightheadedness descended over him from rising so quickly.
"Simple" is your non-branded, straight-shooter "here's what's happening" narrative. This is great: it bascially says "Here is a guy standing up. That's all. Lets move on with the story." Assuming this a mundane action in the grand scheme of your story, this is ideal: just throw your reader a quick action thumbnail and move on.
Now, the "Interesting" one. This is also great, depending on the context. Because here, we're seeing some energy inserted into the scene, some semblance of 'how' things are done, not just 'what'. Now, if it matters that your character (let's call him Kenneth) sat up so quickly that he seemed as though he was electrocuted, then go for it. Perhaps Kenneth has spotted his trusted confidant on TV betraying him before his eyes, or perhaps someone has told Kenneth that someone had once died while sitting in that chair. As long as there is a justifiable reason to give to readers, to say to them "Hey, how this is done is worth knowing," then you're good to go. If there's nothing special about Kenneth's reason for standing up, then stick with the "Simple" route.
"Purple" is far too frivolous for it's own good. It nearly cancels itself out with all of the information it dumps onto the poor reader. Now, if you're thinking to yourself "Hold on, PJ! I quite liked that sentence!", well, you have to put it all in context. You have to look at what you write - really look at it - and think "Is this information necessary?" In what context would the reader need to know the chair Kenneth 'propelled' himself from was a rocking chair? Or that is it old? Why do we need to know that floorboards are wooden?
And it doesn't just apply to descriptions, but actions, too. So this chair screeches across the floor, does it? And poor Kenneth ends up feeling lightheaded from standing up too fast, eh? So what? If it doesn't matter to the story, then cut it. If you have reasons, then by all means go for it: perhaps you are foreshadowing Kenneth's chronic iron-deficiency that comes into play later in the story. In which case, well played. Otherwise, you have this minor action bloated into this mini-action scene that is ultimately meaningless. A pocket-size Michael Bay movie. With rocking chairs and headaches.
And that's the main problem: when you stuff so much of your novel with purple prose of frivolous over-description or actions that go nowhere, then nothing is special. The scenes and moments that truly matter will be drowned out in all of the white noise going on around it. Either that, or you will have to outdo yourself to the point that you will be reaching for the darkest depths of a thesaurus to describe that sweeping sunrise or that scrappy fight between the two anti-heroes, which will elevate it to something approaching parody or pantomime. Toning down the points that don't matter makes the high points easier to stand out.
And isn't that what we're here for, after all? To tell a story? Nobody picks up a book to watch the author perform vocabulary gymnastics, and whenever we as authors do indulge in purple prose the reader finds herself stumbling, her immersion in the world we've created faltering because she has to navigate through the purple prose.
So if Kenneth is going to get out of his chair, tell her that.