What does a blue circle with two arrows mean?
You may pass on either side to reach the same destination.

A blue circle with two white arrows pointing to either side.
What you must do
Keep to the left or the right of the island or obstruction; both are allowed.
Where you will see it
At an island, bollard or obstruction that can be passed on either side.
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.