What does the no stopping sign mean?
No stopping at any time (a clearway). You must not stop, even to set down or pick up.

A blue circle with a red ring and a red cross (X).
What you must do
Keep moving. Only stop in a genuine emergency or if told to by police or traffic signs.
Where you will see it
Fast urban roads and red routes where a stopped car is a serious hazard.
Easy to confuse with: No waiting

No waiting (one diagonal) lets you stop briefly; no stopping does not. See the no waiting 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.