When the top two inputs are true, the bottom input will be inverted. Otherwise, all bits do not change.
This gate is a quantum logic gate, meaning that it can be operated in reverse by having the output as the independent variable and the input as the dependent variable. This circuit does not allow that however, since it is conventional in stead of quantum.
Any Boolean function can be represented by using pure Toffili gates.
Wikipedia page of the Toffoli gate: https://en.m.wikipedia.org/wiki/Toffoli_gate
