Before you take on freelance or second-job work, check your contract's exclusivity clause, then request an NOC confirming your employer does not object. Getting it in writing protects you if the side work is later questioned.
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.