As you create and download files on your Mac your free space shrinks. You can buy a new SSD or go to could storage, or launch Free Disk Space and get long forgotten gigabytes in a moment!
Free Disk Space interactively scans your drives and shows them as a tree-map. It hides small files and folders and makes the rest as big as its size on disk is. If you don't see enough details click on a folder your are interested to zoom in.
You can see the selected file or folder in Finder. When you are ready move it to trash. If you have changed your mind you can move file back with undo.