What does a Bus lane sign with a bus and an arrow mean?
The road at the junction ahead has a bus lane, on the side the arrow points to.

A white rectangular sign with a bus, an arrow pointing left and the words Bus lane.
What you must do
Expect a bus lane as you turn in, and look for the lane markings and the hours plate on the road itself before you use that lane.
Where you will see it
On the approach to a junction with a road that has a bus lane.
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. Signs also appear throughout a full 50-question timed mock, which is the closest thing to the real paper. 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.