It's kind of similar to the syntax of uri query strings and the php syntax for appending to arrays, in that an extra pair of square brackets indicates appending:

In php:

    $a[] = 1;
    $a[] = 2;
    # $a == array(1, 2)

In URIs:

    http://example.org/?a[]=1&b[]=2

Compare to TOML

    [[a]]
    item = 1

    [[a]]
    item = 2