what is the purpose of using square brackets in json?

I am new to json. Some json examples i have seen have data within the curly braces and some json examples have subdata within square brackets.

{
"glossary": {
    "title": "example glossary",
    "GlossDiv": {

        "title": "S",
        "GlossList": {
            "GlossEntry": {
                "ID": "SGML",
                "SortAs": "SGML",
                "GlossTerm": "Standard Generalized Markup Language",
                "Acronym": "SGML",
                "Abbrev": "ISO 8879:1986",
                "GlossDef": {
                    "para": "A meta-markup language, used to create markup languages such as DocBook.",

                    "GlossSeeAlso": ["GML", "XML"]
                },
                "GlossSee": "markup"
            }
        }
    }
}
}

From http://json.org/example.html

What is the need/purpose of having data within the square brackets?

regards

Answer

The square brackets produce a list/array.

The curly brackets produce an object with key/value pairs.

The list can then be a value of a key/value pair.