Often my difficulties in gemara are due to me being thick and/or obstinate and/or lazy. Meaning to say that I don't see things the way the gemara does and I really want the gemara just to look at the world as I do and it's just too much work right now to do the research that will convince me to change. That, of course, can be very frustrating. HaShem, however, "throws me a bone" once in a while and lets me have a really, really good question. The responses I get ("oooo! Good question!" or "I know! I spent a week on that myself.") feed my ego enough to keep me going for a long time. Big egos need very little encouragement, as they are largely self-sustaining, so I only get these great questions once in a while.
The gemara (Gitten 36b), in its discussion of prozbul, takes a small detour to understand why we need prozbul now a days according to Rebbi, who holds that we do not have shmittas k'safim (forgiving of loans in the shmitta year) while we are in exile. The gemara answers that Chazal established shmittas k'safim now a days as a remembrance to the d'oraiso obligation. Hang on one cotton pickin' minute, rejoins the gemara, how can Chazal cancel a loan that the Torah leaves in force!?! (My free translation; not ArtScroll.) Rashi explains the question: since the money is actually -- m'di'oraiso -- owed to the lender, Chazal are turning the borrower into a thief. Abaye resolves the issue, "shev v'lo sa'aseh" -- they are just telling the borrower to refrain from fulfilling the mitzvah of repaying his debt. We are all used to that answer; that's the same reason we don't blow shofar when (the first day of) Rosh HaShanah falls on Shabbos. The Torah says "blow", Chazal say "don't blow"; shev v'lo sa'aseh -- Chazal win.
Hang on one cotton pickin' minute! (That's me this time, not my free translation of the gemara.) Chazal can tell Bob not to fulfill the mitzvah to repay Leon the lender, but Bob still has Leon's money! Isn't Bob still a thief? He has Chazal's blessing, but the thief problem does not seem to have been solved nor even addressed.
It turns out that there is no trivial, "oh... my bad" kind of answer to this question (hence my big ego boost). The easiest way through this is azoi (ie, as follows). The gemara originally thought that there are two reasons that Bob needs to return the money. First, Bob has Leon's money; that's stealing. Second, Bob has a mitzvah of paying off the debt. The gemara answered: That's not how a loan works. When Leon gives the money to Bob, the money now belongs to Bob free and clear. However, Bob has simultaneously accepted upon himself the obligation to repay the loan. That obligation is a real live mitzvah; probably d'oraiso. Moreover, if Bob does not fulfill that mitzvah at the time the loan is due, then he is considered a thief. Technically it is "oshek" and not "gezel", but it amounts to the same thing. Once Chazal tell Bob to refrain from paying, though, both the obligation and the thievery fall away.
This is all needed to even begin to understand about 6 words in Rashi; and we've really only scratched the surface. See why I don't even try daf yomi? I can't even do Rashi Yomi.