Important chemistry question: Can someone please explain to me what a limiting reactant/reagent is?

a limiting reactant is the reactant in a reaction that gets used up entirely first. The excess reactant doesn't get used up entirely, and there'll still be some left over.

Ca + F2 CaF2
The balanced equation tells you that 1 mole of Ca requires 1 mole of F2 in this reqction.
3.00 g Ca / 40.08 g/mol = 0.07485 mole Ca
From the balanced equation you now know that you must have at least 0.07485 mole F2.
2.00 g F2 / 38.00 g/mole = 0.05263 mole F2

You don't have enogh F2, so F2 is limiting.
That means that the amount of CaF2 that this reaction can produce is limited by (dependent on) the available amount of F2.

1.00 mole of F2 will yield 1.00 mole CaF2
0.05263 mole F2 will yield 0.05263 mole CaF2
0.05263 mole CaF2 x 78.08 g/mole = 4.11 g CaF2