What does the no overtaking sign mean?
No overtaking. You must not overtake the vehicle in front until the restriction ends.

A red circle with two cars side by side, one red and one black.
What you must do
Stay behind and wait for the end-of-restriction or national speed limit sign.
Where you will see it
Narrow, winding or otherwise risky stretches of road.
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.