What does the steep hill upwards sign mean?
A steep uphill ahead.

A red triangle with an uphill slope and a gradient.
What you must do
Be ready to change down a gear, and allow for slow vehicles climbing.
Where you will see it
At the bottom of a steep climb.
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. Signs also appear throughout a full 50-question timed mock, which is the closest thing to the real paper. 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.