It refers to a traffic jam. It means you are in a vehicle that is not moving.
The word "sit" can mean "to be in a chair."
It can refer to a posture, a position of the body. You are not standing on your feet. You are not lying on your back. You are sitting on your buttocks.
But it also has some meanings that refer to "staying in one place, not moving, not making progress."
You are always sitting in the vehicle; it is actually the vehicle that is "sitting" in traffic.
"The train sat on a siding for an hour." "
"The plane sat on the tarmac for an hour, waiting for a gate to be available."
"My friends, we mustn't sit idle in the face of climate change, we must take action now!"