That is your recycle bin. Deleted files go into that, it is adjustable in size but generally the size windows chooses is a good one. You can empty it if you are dead certain that you do not need any of those deleted files. It does not grow over the size that is set, the older deleted files become truly deleted as more are put into it.

On rare occasions it can be a lifesaver.

Philadelphia, PA (Really Bensalem)

When you finally understand it completely... it changes.

Hi Dave
Windows settings for the recycle bin are only good for older machines. The default setting is 10% which if you have a newer system with a large hard drive that is a lot of space. I have always set mine to 1% that is still a good amount of space for the amount of times you actually use it.
If you don't know how to do that, just right click on the recycle icon and select properties you will see a slider and you can set it anywhere you like.