What does a triangle showing two lanes becoming one mean?
The dual carriageway ends ahead, so you will meet oncoming traffic.

A red-bordered triangle showing the carriageway narrowing from two lanes to one.
What you must do
Move into the left lane in good time and be ready for two-way traffic.
Where you will see it
Where a dual carriageway returns to a single carriageway.
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.