What does a triangle narrowing on the right mean?
The road narrows on the right-hand side ahead.

A red-bordered triangle showing the road narrowing from the right.
What you must do
Be ready for less room on the right and for traffic merging or coming the other way.
Where you will see it
Where the carriageway loses width or a lane on the right.
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.