I see this selector
$( 'a + .firstSection a' ).live( 'click', function(){
in my js file. What is the purpose of the + sign here?
From the jquery doc:
Selects all the following items matching "next", which are immediately preceded by the brother "prev". A selector corresponding to the element next to the first selector.
Read more about prev + next selector
prev + next
Read it
Selects all the following elements matching "next" that immediately preceded the brother "prev".
Note: even if you do not understand this (and an example!), Now you have keywords to search!
Next time, do not ask questions that can be answered simply by reading the official guide.