I have cars in my unity scene, I am not sure in what units speed is specified in unity.
It's grid units per second.
If you have a frictionless object without gravity at coordinates (0, 0, 0), set its velocity to (1.0f, 0, 0) and wait 10 seconds, it will be at (10.0f, 0, 0).
Regarding what's a "grid unit" in meters: That depends on what you want it to be in your game. In a game where you build a space empire, it could be 1 light-year. In a game where you control a bacteria, it could be 1 µm. But the default assumption seems to be 1 meter. 1m = 1 unit is the scale of most assets you will find on the asset store. Also, the default value for gravity is 9.81f, which would be the value you would expect if your game uses a 1m scale and takes place on the surface of planet Earth.