I try to watch out for bland, overused adjectives and I keep adverbs to a minimum but -- I hear this phrase a lot and I'm not even sure what it means?
I think you're off to a good start by killing the adjectives and adverbs. Overuse of modifiers are the #1 reason I reject a piece of fiction writing.
Ok, how about this excerpt from a marketing email from thriftbooks.com
Please know that this is no whim of fancy but a sincere desire of the heart that is nearly indescribable. Since the day you lovingly wrote your email address on my login page I have kept it indelibly inscribed in the heart (of my database), never to be shared, as the thought of any other having it nearly kills me with jealousy. Just know that I think of it often and desire your love in return. ...
"Purple prose" is overwrought metaphors, melodramatic and clichéd phrasing, and cartoonish actions.
She gasped, her snow-white breast heaving, and her emerald eyes filled with tears.
"How could you! You vile beast!" she sobbed. "I loved you and you — you used me!"
"I never loved you," he announced, cool as a glacier in January. "You were the pleasure of an hour, my dear, and that doesn't include commercial breaks."
Even as he crushed her with his dismissal, she could not help but feel an ache in her throbbing loins to watch him stride across the room like a bored panther. The masculinity rose from his tanned skin in waves, like the scent of the Drakkar Noir she had given him on his last birthday.
"I'm leaving you, Neaveh. I've found... someone else."
"No, Biff!" Her heart-shaped face contorted with grief and jealousy. "It can't be!"
"Don't act so surprised," he sniffed, pulling on his butter-soft leather jacket and slipping on his Ray-Ban sunglasses. "You know I can't be tied down to any one woman. I'm a free spirit. Gone with the wind."
"No — please — don't go — I love you!" she cried, but the door closed with the final, echoing thud of a guillotine, cutting her off from love, from light, from warmth, for her reason for existence. She crumpled to the floor, her fiery red hair spilling around her onto the carpet, and wept until she thought her heart might shatter in her chest.
In summary, don't do that.
In its broadest sense, I would define purple prose as excessive and imprecise description.
Adjectives and adverbs often qualify, as Final Draft illustrated, because a weak adjective + noun combo can be replaced by a single, stronger noun, and a weak adverb + verb combo can be replaced by single stronger verb.
This principle can be applied to almost anything, not just adjectives and adverbs. Look for vague or overly verbose phrases and try to come up with precise, tighter alternatives.