what's the +?

The + ("plus") means a lot.

A LOT. 

Like, a LOT a lot.  Which isn't a bad thing.  In fact, it's a really, really good thing!  

It means that we, as a society, are becoming more inclusive. Hopefully, in more ways than just on paper (or computer screen).

The more different letters, the more different ways that people can identify themselves.  

And, as long as it means we can understand each other a little better each day, we're going in the right direction!

So, again, this is not an exhaustive list.  But it's a start for those that are looking for the information. 

Often the plus stands for:

T, TT or 2: Two Spirit
A: Ally and Asexual
I: Intersexed
A few other words that may be seen, are:
​Cis-gendered
Pansesual