Can someone explain my foundation plan
Those notations (I see 3 on the plan...you’ve circled 2 of them) are in reference to specific footings close to the notations. 

Yes, it is calling out the size of the footing, depth of the footing and the reinforcing steel and it’s location in the footing. It says, 30” square by 18” deep concrete pad with 3 - #4 bars each way in the bottom of the footing.

BTW, the rebar should not actually be on the bottom, but held 3” above the ground. 

Each concrete pad is shown in the plan and the corresponding size and rebar is shown. So, you’ll use the notation circled in orange for the garage and the one circled in pink for the living room...and I don’t see any square pad in the “pink rectangle”.