What does a triangle showing the available height mean?
There is a height restriction ahead, and the safe headroom is shown.

A red-bordered triangle showing the safe height, such as in feet and inches.
What you must do
If your vehicle is taller than the height shown, do not continue; find another route.
Where you will see it
On the approach to a low bridge or structure.
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.