ELSS stands for Equity Linked Savings Scheme. It is an equity fund with one extra property: money you invest in it, up to ₹1.5 lakh a year, can be deducted from your taxable income under Section 80C of the old tax regime.
In exchange, every rupee you put in is locked for three years from the date of that investment. Each SIP instalment has its own three-year clock. This is the shortest lock-in of any 80C option, shorter than PPF, NSC or tax-saving FDs.
Under the new tax regime there is no 80C deduction, so the tax benefit disappears and ELSS becomes just another equity fund with a lock-in. Check which regime you are on before choosing ELSS for the tax reason alone.
ELSS saves tax only under the old regime. Confirm your regime before you lock in.