That's a roll of communication wire. When in a static position it could be rolled out to enable telephone communication between tanks. By not using the radios it reduced the enemy's ability to gather signal intelligence. It also means the tanks don't have to periodically run the engines in order to recharge batteries drained by running the radios.

If you look at tanks like the Centurion and Chieftain, they have a large reel on the rear of the turret as a standard fixture for the same purpose.