When an entity bean is in the pool, it is not connected to a specific EJBObject, and as such cannot perform actions specific to the bean.
It can only perform home business methods, which do not require a specific bean, but involve all beans, such as calculating the average salary for all employees.
Only when an entitybean is brought out of the pool it is connected to an EJBObject, and can then perform actions on that specific bean. This means calling methods that are defined in the component interface, for example update the salary of a (specific) employee.
So the difference is really: in the pool -> bean in not connected to an EJBObject, out of the pool -> bean is connected to an EJBObject