What gets removed
Whitespace between elements, line breaks, indentation and, for code, comments. For JSON, the formatted form with indentation and the minified form {"b":2,"a":[1,2,{"c":null}]} carry exactly the same data.
Why do it
Smaller files download faster and use less bandwidth, which matters for web pages and APIs. For large responses the saving can be noticeable, though it is smaller after compression such as gzip or Brotli is applied.
What it does not do
Minifying JSON never changes values. Minifying JavaScript can rename variables as well, which is a more advanced step that these tools do not perform. Keep the readable original for editing and publish the minified copy.
Try it
Tick minify in the JSON Formatter to compact a document, or turn it off to restore readable indentation. See how to format and minify JSON for details.
Common misunderstanding
Minified is not the same as compressed or encrypted. It stays plain text that anyone can reformat in a second.