Be informed about the details of a situation or task. For example, Don't worry about Sara's taking over that reporter's job—she already knows the ropes. This expression alludes to sailors learning the rigging so as to handle a sailing vessel's ropes. It was being used figuratively by the late 1800s. The same allusion is present in show someone the ropes, meaning “to familiarize someone with the details,” as in Tom's very experienced—he'll show you the ropes.

To be familiar with the details of an operation: “You won't have to train the new computer operator; she already knows the ropes.”