What makes a sentence true? The intuitive answer is the correspondence theory: a sentence is true if it accurately describes the world. "Snow is white" is true if and only if snow is, in fact, white. This is so obvious it feels like a triviality — but spelling it out has been one of the harder projects in twentieth-century philosophy.
Tarski showed how to give a formal theory of truth for limited languages, treating "is true" as a relation between sentences and the world. But for natural languages, correspondence runs into problems: what does "corresponds" actually MEAN? What corresponds to abstract truths like "torture is wrong"? Coherentists (Hegel, Brand Blanshard) said truth is a property of whole systems of belief — a sentence is true if it fits with the rest of what we accept. Pragmatists (James, Dewey) said truth is what works — what survives inquiry.
The contemporary picture is humbler. Deflationists like Paul Horwich argue "true" is just a useful device: to say "snow is white is true" is to say snow is white. There's no deep theory of truth, only a logical convenience. Anti-realists in particular domains (mathematics, ethics) argue you can have a useful concept of truth without metaphysical realism. The fight matters because what you think truth IS shapes what you think disagreement amounts to, and how seriously you take rival worldviews.