What does a blue sign with a bus running against the arrows mean?
A bus lane in which buses travel against the main flow of traffic.

A blue sign with a bus and a down arrow beside two up arrows for other traffic.
What you must do
Watch for buses coming towards you from the opposite direction; do not use the bus lane.
Where you will see it
On roads where buses run the opposite way to general traffic.
Test yourself
The theory test asks you to recognise signs like this one. Try the road and traffic signs short test (with an easy and a hard set), or read the signs revision notes. Source: Know Your Traffic Signs.
Other common road signs
Meaning based on Know Your Traffic Signs and The Highway Code. The sign image is an official Department for Transport sign (Crown copyright) from Wikimedia Commons, reused under the Open Government Licence or the public domain. Always check the current Highway Code on gov.uk. Not affiliated with or endorsed by the DVSA.