What does a triangle with a vertical and a horizontal arrow mean?
Two-way traffic crosses a one-way road ahead.

A red-bordered triangle with a vertical two-way arrow crossed by a horizontal two-way arrow.
What you must do
Watch for traffic crossing or joining from both sides and be ready to give way.
Where you will see it
Where a one-way road meets a two-way road, often at the end of a one-way system.
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.