It’s the way the London Underground is split up, in kinda circles from the centre point. Zone 1 is a fairly small area of central London, zone 2 is a little further out etc. Zone 3 is the closest most people could possibly afford to live. My sister lives in zone 5 and works in Westminster. It’s about a 45 min commute on the tube.
Hey that's not too bad! I was in Zone 3 with a roughly 60 min commute, including walking times to and from the station. Actual train time was probably 35 minutes on the Central line.