You may end up getting a lot of trade offers - more than you have time to respond to. This option allows you to limit the number of trade offers you receive.
Click a number on the Stop allowing trades... field to set the maximum number of trades you can receive from other users. If you select Block all trades, you will not receive any trade offers from other ACC users.
After you reach your trade limit, ACC blocks any further trades until you get around to responding to the trades you have.