What does a red ring with two inward arrows and a width mean?
No vehicles wider than the width shown.

A white circle with a red border and two arrows pointing inwards, with the maximum width shown in feet and inches, and often in metres.
What you must do
Do not enter if your vehicle, including its load or mirrors, is wider than the limit.
Where you will see it
At narrow structures, gateways and bridges.
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.