Learn Italian: Sono io.

In English, we use a placeholder word "It" and the verb "is" ("It is me"). Italian doesn't use placeholder subjects. Instead, the verb simply matches the person being identified. To say "It's me," Italians literally say "[I] am I" (Sono io).

Could I say È me to mean "It is me"?

No, that is a direct word-for-word translation from English that doesn't work in Italian. Always use Sono io. Following this same pattern, if you want to say "It's you," you use the "you" form of the verb and say Sei tu.

Why is the pronoun io after the verb instead of before it?

Normally, if you use a subject pronoun, it goes before the verb (Io sono). But when you want to identify yourself—like when knocking on a door or pointing to yourself in a photo—putting the pronoun after the verb puts the spotlight exactly on who is doing the action.

The lesson says we can usually drop words like io. Why do we need it here?

You usually drop the subject pronoun because the verb sono already tells us it means "I". However, if you just say Sono by itself here, it sounds incomplete, as if you are saying "I am..." and pausing. Adding io completes the thought and emphasizes that you are the one being identified.