URLs can only contain a limited set of characters. Anything else, such as spaces, accents, emoji or symbols, is replaced by a percent sign and hex digits: a space becomes %20 and é becomes %C3%A9. Use Encode (component) for a single value such as a search term or query parameter, and Encode full URL when you have a complete address and want to keep its structure (: / ? # & =) intact. Decode reverses either one.
Percent-encoding writes a space as %20. Some forms use + for spaces in query strings; the decoder here also turns + into a space when it decodes.
A percent sign must be followed by two hex digits. A stray % or a broken sequence such as %E0%A4%A cannot be decoded, and the tool tells you instead of guessing.
See also: Base64 encoder · slug generator · JSON formatter