I am slept.
I am killed by the enemy.
If you’re just sitting there, including having something done to you, you’re the subject. (Absolutive case)
If you do something to someone (transitive verb), you get extra grammar (the ergative case): you are *not* the subject.
In nominative-accusative languages, if you do something to someone, you’re the subject, and whoever has something done to them is the object.
The way linguists explain it is by having S for subjects of intransitive verbs, A for agents of transitive verbs, and O for objects of transitive verbs:
I (S) sleep
The enemy (A) kills me (O)
In nominative-accusative languages, S and A are in the same case (nominative). In ergative-absolutive languages, S and O are in the same case (absolutive).
English actually has an ergative feature tucked away: the –ee suffix. If the verb is intransitive, –ee goes with the subject: I retire, I am a retiree. If the verb is transitive, –ee goes with the *object*: They paroled me, I am a parolee. In ergative-absolutive terms, –ee goes with the noun that would be in the absolutive case.
European languages are nominative-accusative, but a whole lot of languages in the world are ergative-absolutive. A further complication is that some languages are split-ergative: their verbs are nominative-accusative in some tenses, and ergative-absolutive in other tenses.
I said that ergative-absolutive verbs acts as if they are in the passive, which is why “if you’re just sitting there, you’re the subject”. Nominative-accusative languages have a way to flip the subject and object of transitive verbs, and calls that the passive. You guessed it: ergative-absolutive languages have a way to flip the subject and “object” of transitive verbs: “The enemy is is-killed-ed by me = The enemy kills me”. And that mechanism is called… an anti-passive.