A backdoor in a cryptosystem or algorithm allows someone to obtain access to the decrypted data, without the need for the key/password that was used to encrypt the data.

A backdoor in a cryptosystem or algorithm allows someone to obtain access to the decrypted data, without the need for the key/password that was used to encrypt the data.

You can think of a backdoor as a master-key that can unlock all doors, no matter what individual keys were used to lock those doors.