The MAX232 has a built-in voltage doubler and invertor. These are the outputs. You can use them if you want to, but note that they can't supply much current.
1. It means that input/output is Inverted, eg. log. 1 is represented by 0V
2. Yes, it's Schmit trigger input (has different voltage level to flip to 1 then to flip back to 0)
3. It's an output from internal charge pump, you can use it if you need +8V or -8V in your circuit (eg LCD Contrast drive), but only couple mA.