ありがとうございます is the standard polite form of thank you. It is derived from 有り難い, which literally means difficult to exist, expressing that the favor received is rare and precious. The past tense ありがとうございました is used when the favor or event has concluded.
That is how Fuguro works. It brings ありがとうございます back for review right before you forget it, each time a little later, until it sticks for good. You hear it spoken aloud, and you meet it again in stories you can already read. The first five levels are free.