html - What does element#Identifier CSS selector do? -

I wrote some code (or copied more copied) a while ago, and now looking at it .. I Do not know exactly what this CSS selector does:

  div # pop-up  

My guess is that it is saying "if You pop up a div called, I'll choose you ", but I'm not sure. Can someone confirm that this is a valid CSS selector, and if so, what is it doing?

This selects div element equal to "pop-up" With a id attribute

  div # pop-up {color: # f00; font-weight: bold; }  

  & lt; Div id = "pop-up" & gt; Hello World! & Lt; / Div & gt; & Lt; Div id = "not-pop-up" & gt; Hello World! & Lt; / Div & gt;