What does a Bus lane sign with an arrow mean?
A bus lane that runs with the flow of traffic, reserved for buses during its hours of operation.

A sign showing a bus, an arrow and the words Bus lane.
What you must do
Do not drive or park in the bus lane during its hours, shown on the sign; cycles and taxis may be allowed.
Where you will see it
On the nearside lane of busy main roads.
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.