Your sample credits are added to your account each month. When you download a sample, you use a credit. When you download a preset or MIDI file, you use 3 credits. Once you’ve downloaded the sample, future downloads of that sample won’t cost you any credits. It’s yours.
Note that you can “like” as many samples as you want in your online collection without using any downloads. Think of it as a way to bookmark interesting samples and loops that you might want to download later.
Also remember that if you don’t use all of your credits by the end of the month, any credits you have left will roll over to the next month. That means you could save up for a few months, then download enough sounds to create an arrangement of Stravinsky’s the Firebird, using different bird samples for every part. Not that you’d necessarily want to do that, but you could.