What does a sign with a motorway symbol on blue mean?
The start of a motorway, where motorway rules begin to apply.

A blue sign with the white motorway symbol, often with the motorway number.
What you must do
From here, motorway rules apply: no pedestrians, cyclists, learner drivers (except in an approved car), slow vehicles or U-turns.
Where you will see it
Where an ordinary road becomes a motorway.
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.