A list comprises a a number of different items. Often it is useful to address (call for) a list item by its number within the list. For example, historically this was done using .at():
$MyItem = $MyList.at(2);
This returns item number three in the list, as the count used is 'zero-based', i.e. it starts at zero rather than one.
Another, and more current way is to use [N] directly after the object addressed. Thus, and functionally the same as above:
$MyItem = $MyList[2];
This also makes it easier to address values in nested lists. Thus in this list:
$MyList = [4;5;[6;2;4];9;7];
the third list item is itself a list. To address a value within that nested lest, append another set of [] like so:
$MyItem = $MyList[2][1];
returns '2', the first [2] is item #3 in the main list (i.e. 6;2;4) and then [1] calls the second item in that nested list, returning 2.