Help understanding chip function
It's incredibly hard to make anything out on that screenshot. Please try and provide a better one, or a link to the file that it is in.

From general battery charger principles though:


  Does it take USB power and convert it to both vcc_main and vbatt? 


Yes.


  Or does it output vcc_main and vbatt is also an input to the chip?


Yes.

Confused yet? 

If there is USB power then that USB power goes to VCC_MAIN and is used to charge the battery through VBATT.

If there is no USB power then VBATT is used to provide power to VCC_MAIN.