This is nice, but even though I've administered email servers for a quarter of a century, I haven't got the foggiest clue what makes an email server "Gmail-like

This is nice, but even though I've administered email servers for a quarter of a century, I haven't got the foggiest clue what makes an email server "Gmail-like". What does "Gmail server" mean? I would think, if anything, that what Gmail has that typical email servers do not is somewhat decent webmail, but that can't be it because webmail isn't even mentioned. Or is this another one of those instances where people use "Linux" to refer to all things Unix? I genuinely would like to know.