You really don't remember much. I'll break it down.
- Walt made poison for Jesse to kill Gus.
- Jesse hit the poison capsule in a cigarette that he kept with him at all times.
- Jesse decided not to use it.
- Walt had Huell (Saul's bodyguard) stole the poison while frisking Jesse.
- Jesse saw that he was missing his poison and though that Brock must have smoked his cigarette.
- Jesse visits Walt for help.
- Jesse thinks Walt poisoned Brock (He's correct).
- Walt convinces Jesse that Gus did it.
- Jesse and Walt team up and Walt finds a way to kill Gus.
- Afterward, it is revealed that Brock wasn't poisoned with the ricin Walt made, but a common poisonous plant called lily of the valley, which is much less lethal than ricin, which makes Jesse thinks no one actually poisoned Brock, but that he just happened to ingest lily of the valley.
- Walt goes home and we see that there's a pot of lily of the valley in Walt's back yard.
-Walt later tricks Jesse into thinking that he lost the poison cigarette somewhere in his house.
- When Huell pulled the same trick with Jesse's weed, Jesse realized his cigarette WAS stolen and that he's been manipulated by Walt.
So, in short, Jesse knew about the poison because it was his to use. He thought it was for killing Gus, but the ricin was never used.