iMessage is the former name for Apple's Messages app on macOS and iOS. Some people still use the former name as it's a bit more distinct than the current name and/or it's what they're used to. See also iTunes/Music and iCalendar/Calendar, or people who still call macOS "Mac OSX/MacOS X/MACOS X" and so on.
iMessage is Apple's proprietary chat protocol. It's still named that -- for instance in iMessage apps and iMessage stickers. "Messages" is the current name of the user facing app that speaks both iMessage and SMS, which was formerly named "Text" when it just used SMS. I think you're thinking of the defunct iChat message client on macOS.
Actually not quite. iMessage is the protocol/service used by the Messages app to communicate between two iPhones. Conversely when you send a message between an iPhone and any other kind of device it uses SMS.
It’s possible that the GP is unfamiliar with iMessage because they don’t live in NA. I have neither sent nor received an iMessage for several years. I use the Messages app for receiving SMS OTP codes only and pretty much nothing else.
