Banks ask for an employer NOC alongside a salary certificate when assessing a loan. The bank usually specifies the exact wording — attach their template to your request so HR can issue it right the first time.
A short letter from your employer stating they have no objection to something you are doing — travel, study, a second job, a loan application. It is not a legal permission; it is documented awareness and consent.
HR, on company letterhead, with an authorised signature and usually a company stamp. Your line manager's email approval is not normally accepted by embassies or banks.
Typically three to ten working days. Ask early, especially for visa appointments, and give HR the exact addressee and wording the receiving body requires.