An ECS return charge is a fee your bank takes when an ECS debit, such as an EMI or an insurance premium, bounces because the account could not pay it, usually for low balance. ECS debits now run through NACH, so this is the same charge as a NACH return charge, and each bank sets its own amount.
Why it still says ECS
The RBI moved the last ECS centres to NACH by January 31, 2020, but some banks still print ECS on these debits and on the charge for a returned one.
What to do
Check that the debit you missed is paid, because the amount is still due. To avoid the next charge, keep enough balance in the account before the debit date.
Example lines
Made-up lines in the usual shape. Your bank's wording will differ.
ECS RTN CHGS 100000000001Illustrative wording of the fee; your bank's will differ.
Questions
How much is the ECS return charge?
Each bank sets its own, in its schedule of service charges; the RBI has published lists showing how the charges differ between banks.