An adjective is a word that modifies a noun.
For example: the green chair, the old couch, the sexy Marine.
Original sentence: As Kate walked into the abandoned building, she was careful to avoid stepping on any broken floorboards; the last thing she needed was to take a step and end up falling through to the dank and rat-infested basement below.
Without adjectives: As Kate walked into the building, she was careful to avoid stepping on any floorboards; the last thing she needed was to take a step and end up falling through to the basement below.
Not only does it not make sense (Why is Kate worried about stepping on floorboards? How is she going to even avoid that - walk on the walls?), but it takes the intensity out of the situation (sure, falling through the floor to the basement is bad enough, but it'd be 10 times worse to fall through the floor into a basement that was dank and filled with rats!).
However, please note I said adjectives are great when used correctly. Yeah, you know where I'm going with this...
How not to use adjectives.
Using too many adjectives is overkill. It bogs your writing and the story down, making it feel heavy, clunky, and at times, pretentious. You don't need an adjective with every noun. In fact, I'm telling you right now, NEVER use an adjective with every noun! Treat the adjective as you would hot sauce: a little dash of it here and there can spice up a meal and make it better than before, but too much will ruin the meal and make you sick to your stomach. And if you're one of those wacky people who love your hot sauce and put it on everything, well, go get a job at a Taco Bell and sauce it up, baby! But for the rest of you, a little discretion, please.
So what, exactly, does Adjective Overkill look like? Here's an example I found when I was reading stories online one day...
Hands cramping with the unproductive chore, he stood and surveyed the shallow hole, the rocky ground that had never mothered a seed to growth, the only root a dry husk that lie broken at the foot of its shriveled and dehydrated flower, and the setting sun behind the barn.
Count 'em. Go ahead, count 'em That's 8 adjectives. 8 adjectives and only 3 nouns that don't have an adjective with them in one sentence. Adjective Overkill. Not only that, but of those 3 unadjectived nouns, the first one is given an adverb (hands cramping), and the second is given anthropomorphic qualities (ground that has never mothered a seed). It's only the last noun (barn) that is left to stand alone. It's as if the author had suddenly run out of steam and said "Screw it, I'm just gonna leave that one alone; nobody will notice anyway."
But I promise, people will notice. If for no other reason than we've been trudging through your mucky narration for the last three lines and suddenly we find ourselves stepping out of the puddle free and clear. And sadly, that single, simple noun at the end only calls attention to all the overly-adorned nouns in the rest of the writing.
So I beg of you, please, don't let yourself go crazy with adjectives. If you find yourself using more than one or two in a paragraph, consider rewriting the whole paragraph. There are plenty of ways to convey your meaning and setting in prose without the overkill. I promise you. I sincerely promise you. I sincerely, truly, honestly, absolutely promise you.