1. Direct translation: this is the bread that fell to the ground on my jam-covered side. qui est tombée par is a subordinate clause used to modify the preceding antecedent tartine, which is the subject, so qui is used as a conjunction (if the antecedent is the object, then que is used; there are other cases that I won't go into here); the compound past tense form of tomber The auxiliary verb should be être, to match the number of the subject; tomber par terre is a use of the verb tomber to "land", with the preposition par

2. Next time you'll have to put the jam on the other side

I hope this helps.