This is an online "weird text translator". It converts normal text into weird text by using unusual unicode symbols which resemble the normal number and letter characters of the alphabet.
It's able to generate weird text symbols from the normal ASCII symbols that you are able to copy and paste, because all the characters in the right-hand box of the translator are funky characters from the unicode standard.
The weird hacky text is known as "zalgo" text, and is probably what most people are using this translator for. The reason it makes text glitchy is because unicode has a special notation for diacritic marks (those strange little symbols that are above or below characters that you see in French, Spanish, mathematics notations, etc.). Happily, unicode allows us to add as many strange marks to each character as we like, and so we can take advantage of that to convert text into weird hacky tet which you can copy and paste to YouTube comments so people know you're a l33t h4ck3r.
This converter was made with the (partial) help of lunicode.js. Thanks to the creator, @combatwombat.