When the top input is true, the two bottom inputs will swap their states. 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 instead of quantum.
Any Boolean function can be represented with pure Fredkin gates.
Here is a link to a Wikipedia page of the Fredkin Gate: https://en.m.wikipedia.org/wiki/Fredkin_gate
