さ is a sentence-final particle used mainly in casual speech. It adds a sense of certainty or self-evidence, similar to saying "of course" or "I mean" in English. It can soften a statement by making it sound matter-of-fact, or it can slightly challenge the listener, depending on intonation and context. It is common among men and in informal settings, though women also use it in casual conversation. In writing, it appears in dialogue or informal text, not in formal prose.

Be careful: the same spelling さ is also a suffix that turns adjectives into nouns, like 高さ (height) from 高い. That is a different grammar point; here we focus on the particle at the end of a sentence.