Road signs, Queues likely
What does the queues likely warning sign mean?
Traffic queues are likely ahead.

A red triangle with queuing traffic.
What you must do
Slow down, watch for the back of a queue and be ready to stop.
Where you will see it
Before junctions, road works and busy stretches.
Test yourself
The theory test asks you to recognise signs like this one. Try the road and traffic signs quiz (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.