Form 15G (under 60) and Form 15H (60 and above) are self-declarations that your total income is below the taxable limit — so the bank/EPFO/payer does not deduct TDS.
Bank/post-office interest, EPF withdrawal, rent, some corporate deposits.
Only submit if the declaration is true — a false 15G/15H to escape TDS carries penalties. Full detail for provident fund: Form 15G for PF withdrawal.