The supinum (supine) is the verb form used together with har (present perfect) or hade (past perfect) — it never stands on its own. It's invariable: it never changes for gender or number, unlike the perfekt particip (past participle), which does agree when used as an adjective (e.g. en skriven bok, ett skrivet brev) and often looks slightly different (skrivit vs. skriven/skrivet).

Rule of thumb: har/hade + supinum = "have/had done something". Without har or hade, you need the preteritum (simple past) instead — jag har ätit (I have eaten) vs. jag åt (I ate).