It's a little strange, but we've been looking at logical complements already up until now. It's just that here, we'll introduce the idea formally.
Occasionally, we will come across a circuit (which can also be composed of only one gate) that produces the exact opposite of what we want. We can solve this by placing an inverter adjacent to the output of that gate, effectively producing the logical complement of that circuit.
You saw in the previous examples that we just added a circle in front of the gates to denote that we'd be inverting their output- this is basically shorthand for saying that an inverter has been added to the gate.
For this class, we'll only really focus on NAND, but it's good to know how to build NOR as well. We don't particularly find any use for XNOR, so there's no need to worry about it.