What does a triangle showing stones flying from a car mean?
There are loose chippings on the road surface.

A red-bordered triangle showing a car with stones flying up from its wheels.
What you must do
Slow down to cut the risk of a cracked windscreen or chipped paint, and leave extra room behind the vehicle in front.
Where you will see it
On freshly surface-dressed roads.
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.