Escape turns raw text into the inside of a string literal; Unescape reads a literal back. Surrounding quotes are removed automatically when you unescape.
JSON output is exactly JSON.stringify. JavaScript, Java / C# and Python escape quotes, backslashes and invisible characters (\n, \x00, \u2028, lone surrogates…). Unescape also reads \xHH, \uHHHH, \u{…}, \UHHHHHHHH and octal where the language has it. Python \N{name} is not supported.
SQL doubles single quotes (standard SQL; MySQL's backslash escapes are not applied). Shell output goes inside '…': a quote becomes '\''. CSV doubles " and quotes the field when needed. Regex escapes \ ^ $ . * + ? ( ) [ ] { } | /.
Shortcut: Ctrl/⌘+Enter copies the output.