What Liquid Goes in Here?

[elpacero]

I am admitting my total lack of auto knowledge with this stupid question.

Image: ow.ly/i/33kX

The white (well kinda brown since it's dirty) plastic container on left that has the black cap off in the picture is empty. The cap says Engine Coolant Only.

Do I put green coolant / antifreeze in there?

Thanks!

[elpacero]

OK, I'm thinking this must be a overflow tank for the radiator and can be empty?