The phrase ײאין בעד מהײ is not actually in lesson 2
There's a slight fault in the second lesson. The phrase is displayed as one of those learned in the lesson itself on the lesson's icon but when i did the actual lesson all i got was בבקשה‚ (This is because it's a synonym, right?)
in some context they mean the same. if someone tells you "thank you" you can answer בבקשה or אין בעד מה. it's like saying "no problem" or "you're welcome", literal meaning: "there is nothing for" (meaning there is nothing to thank for).