What does a triangle with the word Ford mean?
A ford, a place where a stream crosses the road, is ahead.

A red-bordered triangle with the word "Ford".
What you must do
Slow down and check the depth; the road may be flooded or slippery. Test your brakes gently after driving through.
Where you will see it
On rural roads where a watercourse crosses the 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.