What does the keep left sign mean?
Keep left. You must pass to the left of the sign.

A blue circle with a white arrow pointing down to the left.
What you must do
Move to the left of the island or obstruction the sign is on.
Where you will see it
Traffic islands and the start of a central reservation.
Easy to confuse with: Keep right

The mirror sign means keep right. See the keep right sign.
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.