An ESB architecture introduces a bus to which services and clients connect, instead of connecting directly to each other.
When connecting to a bus, it becomes significantly easier to, for instance, monitor the communication between clients and server in order to perform, for instance, load balancing or failover between instances of a service.
Reducing the direct connections between services and clients also reduces the impact of changes.
If, for instance, a service changes its data format, an adapter that transforms the data from the new format to the format expected by the (old) clients.
There is of course much more, but I hope that you have got at least a hint on what an ESB can be.