See exactly which bytes a string occupies.
UTF-8 stores ASCII characters in one byte and uses two to four bytes for everything else. The tool shows those bytes in hexadecimal, decimal, binary or percent encoded form so that you can inspect them, count them or paste them into code.
To decode, paste bytes in the chosen format. Every value must be between 0 and 255, and the sequence must form valid UTF-8, otherwise a specific error explains the problem.
One for ASCII, two for most European accented letters, three for most Asian scripts and the euro sign, and four for emoji and rare characters.
This shows the encoded bytes. The Unicode Text Converter shows the code points, which are the abstract numbers assigned to characters.
That is mojibake, caused by reading UTF-8 bytes as another encoding. Decoding the bytes as UTF-8 here shows what the text was meant to be.