What does the give way sign mean?
Give way to traffic on the major road. You do not have to stop if the road is clear.

An inverted (upside-down) white triangle with a red border.
What you must do
Slow down, be ready to stop, and only emerge when there is a safe gap.
Where you will see it
The minor road at a junction, often with a double broken white line across the road.
Easy to confuse with: Stop

A stop sign requires a full stop every time; give way does not. See the stop sign.
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: Highway Code rule 172.
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.