Answer from cs61c-ew (Joo-Rak Son 16103505) for Question 1
It is a slot in pipelining scheme between the execution of branch instruction and the following instruction (which is determined by the branch instruction), in which an unrelated instruction is inserted.
Without the utilization of this slot, the processor will be much slower because it has to stall until the next PC is determined according to the branch instruction. By inserting something unaffected here, we save time and achieve normal pipeline speed.