What do runway numbers mean?
A runway number is its compass direction, divided by ten and rounded. Runway 27 points roughly west.
Reading the number
Take the runway’s magnetic compass heading, drop the last digit and round. A runway pointing 270 degrees (west) is runway 27. One pointing 090 degrees (east) is runway 09.
Each runway has two numbers, one at each end, that differ by 18. The same strip of tarmac is runway 09 one way and runway 27 the other.
Left, right and centre
Where an airport has parallel runways, a letter is added: L for left, R for right and C for centre, as seen from an aircraft approaching.
Which one is in use?
It depends on the wind. See “Why do planes take off into the wind?”. The airport can change runway during the day as the wind turns, which is why the weather report on an airport page matters.
Watch for it
On AirLive the runway in use is shown from the latest weather report, and aircraft in the Inbound and On the ground lists are matched to it.