Cable is the sterling/US dollar exchange rate.
The vast majority of currencies are quoted in terms of the amount of the other currency to the dollar. Cable is an exception and is quoted in terms of the number of dollars to the pound. The other major exception is the Euro.
The term originates in the use of transatlantic telegraph cables for forex trading, the previous dominance of the pound as the major currency for international trade (and therefore foreign exchange transactions), and a great deal of inertia in market conventions (the result of the needs for clarity: misunderstandings by traders are very expensive).