What is "Reishisam" referring to?
Rashi: It is referring to Terumah Gedolah. 1
Which is called "Reishis", because it precedes Ma'aser Rishon (Rashi in Eruvin, 31b). .
Why does the Pasuk insert the word "Cheilev" (the best) by each of the fruits?
Rashbam: This is based on Pasuk 30, where the Torah writes that one should give "Cheilev" when separating Terumah.