It creates a branch recorder and attaches it to the processor, just as if you had typed
new-branch-recorder branch_recorder physical
cpu.attach-branch-recorder branch_recorder
The branch recorder will (as the name implies) record branches taken by the processor (by physical addresses); various useful numbers can be computed from them. You can for example display execution count statistics by typing
branch_recorder.address-profile-data
(but you will not see anything interesting until you have let the processor run a few instructions).
With the -virtual flag, record virtual addresses; otherwise, record physical addresses.