What does a triangle showing a long vehicle on a hump mean?
Long, low vehicles risk grounding on a hump bridge or level crossing ahead.

A red-bordered triangle showing a long, low vehicle grounding on a hump.
What you must do
If you drive a long, low vehicle, take care, as the ground clearance may not be enough.
Where you will see it
At steep humpback bridges and some level crossings.
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.