What’s the meaning of this sign?

Explanation: This sign indicates that there's a tunnel ahead. You should always use dipped headlights in a tunnel - regardless of the time of day - and may need to lower your speed as you enter. Your eyes may need a moment to adjust to the lower level of light.

Category: Road and traffic signs (2021)

References: The Official DVSA Guide to Driving: Section 6, Highway Code: Page 108, Know Your Traffic Signs: Page 12