了 is a picture of a swaddled infant with its arms wrapped and no legs showing — a sense of something 'wrapped up / finished'. Read liǎo it means 'to finish / to understand'; toneless as 'le' it marks a completed or changed action.
了 is how Chinese says 'it happened'. Chinese has no past tense — instead you add 了 after a verb to show the action is complete: 吃 (eat) → 吃了 (ate / have eaten), 来 (come) → 来了 (has come), 好 → 好了 (it's done / ready now). It also marks a new situation: 我好了 = 'I'm better now.' One little hook does the work of English tenses.
A swaddled baby, all wrapped up — something FINISHED. Add it after a verb: it happened.
Chinese has no verb tenses. Instead of conjugating, you just add 了 to mark 'done': 来了 (came), 走了 (left), 买了 (bought). Simple — and 了 is everywhere.