JSON Schema validation answers one question: does this document satisfy the contract. The useful part is not the yes or no, but the explanation when the answer is no.
Every validation error has three components worth reading. The property path tells you where in the document the problem is, written as a pointer like /items/0/name. The keyword tells you which constraint was not met — type, required, enum, additionalProperties. The actual value tells you what was there instead. An error missing any of the three is hard to act on.
A handful of keywords do most of the work in practice. The type keyword catches the string-where-a-number-was-expected case. The required array catches missing fields. The enum keyword constrains a value to a fixed set, which is the single most effective way to stop a model inventing a category. And additionalProperties set to false catches fields nobody declared.
Validate the schema itself before blaming the document. A schema with a typo in a keyword name often still compiles and simply fails to constrain anything, so everything passes and you conclude the validation is working. Compiling the schema separately from validating the document distinguishes the two failures.
Where validation runs matters as much as whether it runs. Validating in the browser means the schema and the document never leave the machine, which matters when the document is a customer record or the schema describes something unreleased. It also means you can check a payload without waiting for a deployment.
One caveat on drafts: JSON Schema has several, and they differ in ways that occasionally matter. Draft 2020-12 is the current default and the one most provider tool-definition formats expect. If your schema declares a $schema keyword, whatever validator you use should respect it.