What does a blank mean?

This question has already been asked and here is my answer to it

_ is called the underscore.

The underscore is treated by Lua exactly like a letter (a-z, A-Z). It is used in variable names, or on its own (just like you could use x as a variable name, you can use _ as a variable name).

Since spaces aren't allowed in variable names, some styles suggest using the underscore as a space, as opposed to using camelCase or PascalCase: multi_word.

In addition, some built in Lua names use it, including _G, the global table, and the metamethods, which are preceded by two underscores, e.g, __index.

Usually, when _ is used as a variable name, it is to indicate that its value is not important.

Since some functions, like the iterators pairs and ipairs, return two values, you need to give a name to the first value even if you aren't going to use it. Stylistically, the _ is sometimes used to note that.

It is only a stylistic convention and is nothing special, though.