The term Enterprise Service Bus borrows from the Bus that connects CPU, RAM and other chips.
The ESB is a specialized subsystem dedicated to communication to other subsystems.
This approach allows to cut the number of interfaces between the systems. Thus, a subsystem (a service or a consumer) can be exchanged with another subsystem more easliy because there are not many dependencies.
This design is apparently also known as horizontal integration.
Tyipcally, an ESB is implemented as a server or group of servers.
An intelligent service bus can even coordinate distributed transactions.
It is also possible to delegate authentication and authorization to the service bus.
With all these advantages, it must not be forgotten that the service bus is a single point of failure.
Also, if the ESB does not perform well, it is a bottleneck for all applications connected to it.