A broadcast is the simultaneous transmission of the same data to every device in the network regardless of whether the device needs it or not.
A broadcast domain is a set of devices that receives a copy of broadcast sent by a device within that group.
Hubs and switches floods the broadcast on all of their interfaces therefore they create a single broadcast domain. On the other hand, a router breaks a broadcast domain.
Every device on the network has to process received broadcast, so we need to control the size of the broadcast domain. As the size of a broadcast domain increases, efficiency of the devices decreases. Routers do not forward broadcasts therefore they help to breaks that broadcast domain. A broadcast sent by a device within that broadcast domain is not forwarded to another broadcast domain.
In figure, there are two broadcast domains. The router does not forward a broadcast sent by any host in HUB network to SWITCH network and vice versa.