What does a triangle showing an arched bridge mean?
There is an arched bridge ahead, lower at the sides than in the middle, with the headroom shown.

A red-bordered triangle showing an arched bridge with the maximum headroom.
What you must do
High vehicles must keep to the centre of the arch, where it is highest, and obey the height shown.
Where you will see it
On the approach to an arched railway or canal bridge.
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.