What does the no U-turn sign mean?
No U-turns. You must not turn round to go back the way you came at this point.

A red circle with a U-turn arrow crossed by a red diagonal line.
What you must do
Carry on and turn round somewhere safe, such as a roundabout or a side road.
Where you will see it
Central reservations and junctions on busy 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.