How to use "this" in a selector expression?

How to use "this" in a selector expression?

Can I use "this" in a selector expression? Something like

$("this > childrenelement").....

I did see here something like:

$(' > img', this)
But where is this documented/explained better?

Since "this" is a javascript keyword and not just a jQuery keyword, it would be interesting to have an answer from both jQuery and Javascript point of view.