What does a red circle with a weight in tonnes mean?
No vehicles over the maximum gross weight shown, often to protect a weak bridge or a quiet area.

A red circle with a maximum weight in tonnes, for example 7.5T.
What you must do
If your vehicle is over the limit, take another route. Cars are not affected.
Where you will see it
Weak bridges and roads with a weight restriction.
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.