On February 16th, 2016, the FBI demanded that Apple "assist" with an ongoing investigation of a terrorist attack.
The FBI's request was for Apple to change iOS (the iPhone's operating system) to contain a security flaw, which would enable the FBI to unlock (thusly decrypt) the iPhone in question.
As pointed out by an open letter from Apple's CEO, Tim Cook, creating such a "backdoor" is an extremely dangerous premise. There is no way to guarantee that such an exploit would only be used by law enforcement in appropriate situations. If the exploit is there, it's open for everyone.
This would harm only users who have no malicious intent by deprecating their privacy. Those who intend to commit crimes would trivially use alternate tools.
This is an unreasonable breach of civil rights.