はい is the most common word for yes or certainly. Beyond agreement, it is frequently used as a backchannel (あいづち) to show the listener is paying attention. In formal settings, it can also mean I understand or Present (during roll call).
That is how Fuguro works. It brings はい 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.