What does the no cycling sign mean?
No cycling. Cyclists must not ride past this sign.

A red ring with a black bicycle inside it, with no diagonal bar.
What you must do
If you are cycling, get off and push, or take another route.
Where you will see it
Pedestrian areas and footpaths.
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.