In order of frequency: a trailing comma after the last array element or object property; single quotes instead of double quotes; unquoted object keys; comments (// or /* */), which JSON does not support; and unescaped newlines or double quotes inside a string. The sample input above deliberately contains a trailing comma so you can see the error report.
Does this validate against a JSON Schema?
No — this checks syntax, not structure. It answers "is this parseable JSON?" rather than "does this match my expected shape?". Schema validation requires a schema document and is a different job; this tool reports the root type, nesting depth and key count so you can sanity-check the shape yourself.
Why does my JSON validate here but fail in my application?
Usually one of three things: your application expects a specific schema rather than any valid JSON; the file has a byte-order mark (BOM) or trailing content that your HTTP client includes but a copy-paste drops; or duplicate keys are present, which is technically valid JSON but which different parsers resolve differently.