A breaker bar is typically used to give the user extra leverage to break loose a stubborn or high torque fastener.
A typical breaker bar, such as the GearWrench 81308D 1/2" drive breaker bar in the Amazon Affiliate link box to the right, is a long metal rod with a socket attachment, which is usually not a ratcheting head because they are less durable than just a direct connection. That durability is important, because a breaker bar is designed to deal with a lot of torque. This particular example is a 1/2-inch drive model, which allows for the use of 1/2-inch drive sockets to be used with it. It also has a 24 inch handle; the longer the handle, the more potential leverage the user has.
The Downsides of Breaker Bars
A breaker bar is often the go-to tool for loosening a fastener that appears to be stuck (such as a rusted bolt), but it offers a level of confidence that could cause the user to break the fastener loose when other methods of loosening the fastener (such as heating it or spraying penetrating fluid) would have improved the odds of removing the fastener without breaking it.