What does the turn left sign mean?
Turn left here. You must follow the direction of the arrow.

A blue circle with a white arrow pointing left.
What you must do
Turn left at the junction.
Where you will see it
Junctions where you must turn left.
Easy to confuse with: Turn left ahead

Turn left ahead warns of a left turn coming up; this one is at the turn itself. See the turn left ahead sign.
Test yourself
The theory test asks you to recognise signs like this one. Try the road and traffic signs quiz (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.