是 stacks the sun 日 above a foot walking a straight line: what is direct under the sun is upright, correct, so. From 'correct' it became the copula — the little verb 'to be' that welds every A 是 B sentence in the language.
是 answers questions the way English says 'yes': 是不是？是。Its philosophical weight is real too — 实事求是 'seek truth from facts' hangs at Party schools, and 是非 'is-and-isn't' means right-and-wrong. To know 是非 is to have judgment.
Standing straight under the sun 日 — what IS, plainly and correctly.
是 appears in roughly one of every twenty Chinese sentences — learn this one character and you've met 5% of all text you'll ever read.