In some languages it may be convention to use the underscore (_)
for special purposes at the start of an identifier, for example
I've seen people use the underscore to indicate local variables.
In your example, it looks like whoever chose the name '$item'
for their variable may by a PHP programmer who did it out of
habit, or something. In PHP, all variable names must start with
a dollar sign. Interestingly, having an identifier that is just
a dollar sign, and nothing else, is something that jQuery does
for its topmost-level object. It would be a good idea, if jQuery
was the only thing in the world to use that identifier name. But
it isn't, and therefore it isn't (a good idea), which you'll
discover when you try to combine jQuery and Prototype, or any of
the other libraries doing the same. That said, jQuery is a good
library and it is not too difficult to make it use some other
name than '$' if you need it to share with another library.