What the heck does that mean: "Log in with https?"

Could not find a definition/explanation. Would that change my log in ID? my e-mail address? the e-mail address I tell others?

The only time I see https is before "www" to indicate a web page. I've never seen an e-mail address with "www" so how/why would you use https?

(If it's so important, why isn't it explained?)