What does a sign showing no loading mean?
No loading or unloading during the times shown on the plate.

A yellow rectangular plate reading No loading, with a blue disc inside a red ring and a red diagonal across it.
What you must do
Do not stop to load or unload here during the restricted times.
Where you will see it
On busy roads and clearways where stopping would block traffic.
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.