None of the other answers actually answer the question.
This is percent sign notation. The percent sign indicates that the next character is a literal delimiter, and you can use any (non alphanumeric) one you want. For example:
%{stuff}
%[stuff]
%?stuff?
etc. This allows you to put double quotes, single quotes etc into the string without escaping:
%{foo='bar with embedded "baz"'}
returns the literal string:
foo=’bar with embedded “baz”‘
The percent sign can be followed by a letter modifier to determine how the string is interpolated. For example, %Q[ ] is an interpolated String, %q[ ] is a non-interpolated String, %i[ ] is a non-interpolated Array of Symbols etc. So for example:
%i#potato tuna#
returns this array of Symbols:
[:potato, :tuna]
Details are here: Wikibooks