Cable is the forex market's nickname for the GBP/USD currency pair, the exchange rate between the British pound and the US dollar.
The name comes from the 19th-century transatlantic telegraph cable that carried exchange rates between London and New York. Traders still call GBP/USD Cable today.
Cable moves on Bank of England and Federal Reserve policy, UK and US economic data, inflation, interest-rate expectations, and global risk sentiment. It is one of the most actively traded major pairs, so its spreads are typically tight.
You see GBP/USD = 1.2700, so one British pound equals 1.2700 US dollars.
If GBP/USD rises to 1.2800, traders say Cable has risen. If it falls to 1.2600, they say Cable has fallen.