What is coaching according to PM?

coaching is done by manager to improve effectiveness and performance of the employee. Manager can give his/her experience to teach his subordinates and make them learn from his/her success/failures. Plus with the PMI-PMP certification manager can teach good practices to follow. This helps team to upgrade their knowledge and apply it to their work and produce good results.

Coaching is continuous, but is most required when team members commit mistakes, not working upto the expectations, are disoriented, are feeling low, etc, etc

Yes, it should be (can be) for everyone as no one is perfect and there is always something to learn everyday. even the manager can learn new things from his juniors.