$+

The identifier $+ is used to combine (concatenate) items together when it's impossible just to write them together due to variables or identifiers.

$+ OR $+(...)

The identifier $+ can be used two different ways. One way is just as $+ between items and another one is by passing arguments such as $+(item1, item2, ..., itemN)

here we have so $+ me text  ;returns here we have some text
$+(here,we,have,some,te $+ xt)  ;returns herewehavesometext

When used that way, the identifier doesn't look too useful. But when we have a variable or identifier as an item, it's necessary to use either of these $+.

msg $chan You are $nick ( $+ $fulladdress $+ ) could return You are Dana (Dana!dana@script.quakenet.org)
msg $chan You are very smart $+($nick,;) welcome to $chan $+ . could return You are very smart Dana; welcome to #brainiacs.

Notice how if we had used $chan. or $nick; mIRC would look for identifiers with these names and return nothing.