Cable is one of several slang terms for various currency pairs, in this case the British pound sterling versus the US dollar. This can also be displayed as GBP/USD or GBPUSD. The price of the British pound is sometimes referred to as cable.
The nickname GBP/USD comes from the cable that runs beneath the Atlantic Ocean and was used to transmit the exchange rate between the US dollar and the British pound. Despite the fact that technology has rendered the cable obsolete, the name has stuck.
Other currency pair nicknames include the loonie, which refers to the Canadian dollar versus the US dollar, and the Swissy, which refers to the Swiss franc versus the US dollar. Because the US dollar is the most commonly used quote currency, the majority of nicknames are for currencies against it.