This is the 3rd most frequent Polish word.
się
"Się" is a reflexive pronoun used to indicate that the subject of the verb performs an action on itself or for impersonal/passive constructions.
Reflexive pronoun indicating the action affects the subject.
Uczeń przygotował się do egzaminu.
The student prepared himself for the exam.
Used to express a mutual action among subjects.
Spotkajmy się jutro.
Let's meet tomorrow.
Indicates an action that occurs without direct subject involvement, reflecting a kind of passive voice.