What does the two-way traffic warning sign mean?
Two-way traffic ahead. You are about to meet traffic coming towards you, often after a one-way section.

A red triangle with two vertical arrows, one up and one down.
What you must do
Keep left and be ready for oncoming vehicles.
Where you will see it
Where a one-way street or dual carriageway returns to two-way traffic.
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.