先生is a respectful title used for teachers, professors, and professionals like doctors or lawyers. The kanji literally mean one who was born before. It can be used as a standalone title or as a suffix after a person's surname.
That is how Fuguro works. It brings 先生 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.