What is the "departure cable" that tower mentions?
The "departure cable" refers to an arresting gear cable on the departure end of the runway, similar to the one used for landing on aircraft carriers. Airports that often have fighter jet traffic may have an arresting system installed near each end of the runway.

Fighter jets tend to take off and land at higher speeds than other aircraft, and don't have the same braking power that other jets do. The speed brakes may not provide substantial drag, and the engines typically do not have reverse thrust capability. Since there is more room on a runway than on an aircraft carrier, land-based systems take more distance to stop aircraft.

The system is not used for every landing, but is useful to have in certain situations. For normal landings or aborted takeoffs, the system on the departure end will be used to prevent a runway overrun. If an aircraft needs to make an emergency landing, it will use the system closest to the approach threshold. Some cable systems are able to retract into a recess in the runway when not in use, which prevents issues when aircraft roll over it.

When the system is deployed in normal use, the tower will advise aircraft that the departure end cable is up. The departing aircraft will generally be in the air by the time it reaches the cable, but in case of a late rotation or aborted takeoff, the pilots can anticipate running over the cable.

Arresting gear is indicated on the FAA airport diagram as follows:


Source

The cable is marked by 10-foot diameter yellow circles on the runway.


Source: Google Maps