What does a red ring with a lorry and a weight mean?
No goods vehicles over the maximum gross weight shown.

A white circle with a red border showing a lorry and a maximum weight, such as 7.5 tonnes.
What you must do
If your goods vehicle is over the weight shown, do not enter unless a plate names an exemption such as access for loading.
Where you will see it
On roads and in areas 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.