What exactly ExecutionId signifies?

What exactly ExecutionId signifies?
Can a ProcessInstance have multiple executionIds?
If so , when they are changes , Is it on execution of any async task?
Please give some insights.

These concepts are explained as part of the RuntimeService here in our documentation.

Basically an executions are:

Executions are a representation of the ‘token’ concept of BPMN 2.0. Basically, an execution is a pointer to where the process instance currently is

So a process instance might have one ore more executions depending on the flow.