(The Bay Area contingent of HN can answer that!) Traffic lights (typically red/green only) at the bottom of on-ramps that rate-limit ("meter") traffic onto the freeway, typically during rush hour. You drive up to a red, stop, it turns green and you go. (Sometimes a sign indicates that 2 cars get to go per green given.) There are usually sensors embedded in the rightmost lane of the freeway shortly before the merge point, and on a good day, it feels like it gives you a green timed with a gap. (Although I think it also sometimes just times out and lets you go, in which case, no gap.)
The ones here have an HOV lane and a non-HOV lane, usually; typically they just force the HOV lane to come to a near stop before turning green; I think that's more to slow the HOV traffic down to avoid collisions with the adjoining non-HOV lane should their light also turn green, but it feels like a weird formality when driving it. Some days the cops sit on the ramp pulling people cheating in the HOV lane.
https://en.wikipedia.org/wiki/Ramp_meter
It is one of the two things I warn family visiting that have never visited before. (In the majority of the US, you just get on the highway, there isn't a traffic light at the bottom of a ramp, that'd be absurd, since you're wanting to accelerate to ~70mph.) (The other is lane splitting: motorcycles will (ab)use the dividing line, particularly between lanes 1 & 2, to pass.)