In Lua, a string is any collection of characters. It can be enclosed in single quote characters, double quote characters, or pairs of square brackets:
"Dr. John Smith" 'Radiology' [[Downtown Hospital]]
You can use square brackets to enclose multiple lines:
[[The results of the lab tests are in. No abnormalities have been detected.]]
If you want to include brackets in a multi-line string, you can use an open bracket [ followed by one or more = characters and then another open bracket to indicate the start of a string. The string is ended by a close bracket ], the same number of = characters, and another close bracket:
[==[This string contains  bracket characters, but is ended by a ], two = and another ].]==] [=[This string also contains  brackets]=]
For more about string concatenation see: the Concatenation section in the Lua reference manual, the online programming book also concatenation is covered in the strings tutorial of the Lua Tutorial wiki.