What does the steep hill downwards sign mean?
A steep downhill ahead. The figure shows how steep the gradient is.

A red triangle with a downhill slope and a gradient, for example 10%.
What you must do
Choose a lower gear before the hill so engine braking helps control your speed.
Where you will see it
At the top of a steep descent.
Easy to confuse with: Steep hill upwards

The matching sign warns of a steep climb instead. See the steep hill upwards 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 160.
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.