What does a blue circle with a cycle and people together mean?
A route shared by cycles and pedestrians, with no separate sides.

A blue circle showing a cycle and people together, with no dividing line.
What you must do
Cyclists and pedestrians share the space, so cyclists should give way and pass with care.
Where you will see it
On shared-use paths and some pavements.
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.