What does the level crossing with barrier warning sign mean?
A level crossing with a barrier or gate ahead.

A red triangle with a gate.
What you must do
Be ready to stop. Never cross when the lights show or the barriers are down.
Where you will see it
Where a road crosses a railway with barriers.
Easy to confuse with: Level crossing without barrier

A crossing with no barrier shows a train instead of a gate. See the level crossing without barrier sign.
Test yourself
The theory test asks you to recognise signs like this one. Try the road and traffic signs quiz (with an easy and a hard set), or read the signs revision notes. Source: Highway Code rule 293.
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.