I suppose it is very difficult do define "truth" in an informative way (without just giving a synonym or something like that). Can you explain why it is so? Or is it easy?

One reason that it's difficult to define "truth" might be that the word stands for a concept that's too basic, too fundamental, to be informatively defined in terms of other concepts. I myself think that truth is a property of some propositions and therefore, derivatively, a property of some sentences. Which propositions? The true ones! Which sentences? The ones that express true propositions! The proposal that we can't say more than that is sometimes known as "deflationism" about truth. For much more, see this SEP entry.

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