The contract makes a low-level call to another contract
The call returns a boolean with the result of the operation, true if it succeeded and false otherwise. In this case it is discarded because it is never assigned, so the result of the operation is ignored.
As a security measure EVM reserves some gas ...
The challenge with token transfers is that you can't detect them by monitoring the "receiving" address. Tokens are not explicitly transferred to an address - they are only assigned to an address inside the token contract. So whenever someone transfers tokens to address A he actually sends a transaction to the token contract at ...