What does the give priority to oncoming vehicles sign mean?
Give priority to oncoming vehicles, because the road is too narrow for two to pass.

A red circle with a tall arrow up and a shorter red arrow down.
What you must do
Wait and let oncoming traffic through before you go.
Where you will see it
Narrow stretches and narrow bridges.
Easy to confuse with: Two-way traffic

This is an order to give way; the two-way traffic sign is only a warning. See the two-way traffic 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: 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.