Cable is one of a few slang terms for different currency pairs; in this case referring to British pound sterling against the US dollar. This may also be shown as GBP/USD or GBPUSD. Occasionally, people also refer to the price of the British pound as cable.
GBP/USD got its nickname from the cable that runs under the Atlantic Ocean, which was used to transmit the exchange rate between the US dollar and the British pound. Despite technology making the cable redundant, the name has stuck.
Other currency pair nicknames include the loonie, for the Canadian dollar against the US dollar, and Swissy, for the Swiss franc against the US dollar. The majority of nicknames are for currencies against the US dollar because it is the most commonly used quote currency.