Encryption with an asterisk.
It is becoming increasingly common for companies to encrypt user data in a way that prevents anyone (including law enforcement agencies) from accessing that data. For example, Apple has a long running dispute with the FBI about whether Apple can be compelled to "unlock" iPhones.
The "backdoor" proposal is that companies would continue to encrypt user data, but they would be forced to leave open some secret "door" that only law enforcement had the "key" to. This would allow law enforcement to unencrypt data if they needed to for an investigation.
The idea of building encryption backdoors is very controversial and broadly unpopular among industry security experts. Former FBI Director James Comey has argued that strong encryption has frustrated law enforcement’s ability to conduct investigations.