What does a blue circle split between a cycle and people mean?
A route for cycles and pedestrians with separate, marked sides.

A blue circle divided by a line, with a cycle on one side and people on the other.
What you must do
Cyclists keep to the cycle side and pedestrians to the other; stay on your side.
Where you will see it
On shared paths that are divided into a cycle lane and a footway.
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.