Turn text into a URL-safe form (percent-encoding) and back. Use component mode for values inside a link, full-URL mode for a whole address.
URLs may only contain a limited set of characters, so anything else is written as %XX codes. encodeURIComponent (component mode) also encodes / ? & and = — correct for a value going inside a query string. encodeURI (full mode) leaves the structural characters of a URL intact and only encodes the unsafe ones.
Why did decoding fail with an error? A stray % that is not followed by two hex digits makes the whole string invalid. The tool reports it instead of guessing.
Is a space %20 or +? In a URL path, %20. In form-submitted query strings, + also means space. Component encoding here produces %20.