What does a sign with a diversion arrow mean?
Traffic is diverted along the route the arrow shows.

A rectangular sign with a diversion arrow, usually black on yellow.
What you must do
Follow the diversion arrows around a closure; a symbol may be used to mark the route.
Where you will see it
Around road works and road closures.
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.