What it does
Put the document in the first box and a JSON Schema in the second. The problems are listed as you type: where in the document each one is (a JSON Pointer, or (document) for the whole of it) and what is wrong. Hover a problem for the schema keyword that said so, and where that is in the schema.
Which draft
The draft is read from the schema's $schema: drafts 4, 6, 7, 2019-09 and 2020-12 are understood, and the newest is used when there is none. The status line says which was used.
Formats
Check formats holds strings to their format (email, date-time, uuid and so on). Recent drafts leave that to the validator, and most people expect it, so it is on.
Nothing is fetched
A schema is something you paste in, and it must not make a page read a URL: a $ref has to point inside the schema (#/$defs/name, an $id or an $anchor in it). A reference to somewhere else is reported as a problem with the schema. At most a thousand problems are listed.
In your tab
The check is made by the same Rust engine as the desktop app, compiled to WebAssembly: nothing is uploaded. The module that does it is a little over 400 KB, and is loaded on this page only. Up to 10 MB a box here. To tidy a document first, use Format.