Confusingly, this can mean two things. First, it can refer to the bus arrival times display on bus shelters. 

Second, it can refer to a special aspect on traffic signals which tells pedestrians how long they have left to cross after the green man goes out (known as "blackout"). When used on a standalone pelican crossing, the flashing green man and amber signal (to traffic) is removed so pedestrians see the countdown clock and drivers see a red signal. Countdown is not yet approved for toucan crossings.

The use of the technology is controversial. One the one hand, pedestrians like the crossing (PDF link) time certainty compared with blackout. On the other, it has been suggested that the roll out in London has been used as a veil under which to tweak crossing times down and thus making people feel rushed. Countdown also means the loss of puffin-style detection which will mean that the red signal to traffic doesn't get extended to cater for slower pedestrians.

In the Netherlands countdown is used in an entirely different way - to inform people waiting to cross, either on foot, or on bike, how long they have to wait for their next green phase. The intention is to cut down on risky red light jumping. Countdown to green, for pedestrians and cycles