The slow growth or increase of something is a buildup. Over the course of a winter storm, there will be a buildup of snow on the car that's parked in your driveway.
The process of something collecting or growing is a buildup, and so is its result. As you buy more and more things with a credit card, you'll see a buildup of debt. And If you venture into your uncle's spooky attic where no one's dared to go for years, there's bound to be a buildup of dust on every surface. Buildup has also come to mean "increasing excitement, preparation, or publicity," like the buildup before Christmas at the mall.