Traffic on road ahead is being controlled by portable light signals

What it means

This faces you as you wait to come out of a side road: the arrows mean the road across your path is under temporary portable signals, in both directions. Those lights are not controlling your junction, so nothing is holding the main road for you and normal priority at your exit still applies. Expect the road to fall quiet and then deliver a bunch of vehicles at once as each green releases them, and expect the queue to sit across your exit at times, so wait for a gap that is genuinely big enough.

Highway Code Rule 288 — read it on GOV.UK