Jami and general telephony

I apologize for being ignorant. I didn’t find a sticky thread, and the documentation I’ve found addresses implementation, not description, use-cases, or architecture.

What is Jami? Can it be used for general telephony? Is there a bridge to the public telephone systems so that Jami can be used as a telephone? Are there any companies that are providing the bridge? If not, is that being worked on? If not, is that being comtemplated?

Thanks, and I applaud what Jami aims to do.

PS: An article about MALOYA mentions “This feature [SIP protocol support] brings the ability to place and receive calls with regular phone numbers.” I don’t know what a SIP phone is, or what a SIP-compatible telephone system or service provider is, but I’d like to know more.