[ ] — Literal / Index / Axis[A ⋄ B] assembles cells with fill. Newlines also separate cells.
[1 2 ⋄ 3 4] ⍝ 2 2⍴1 2 3 4
[1 2 ⋄ 3] ⍝ 2 2⍴1 2 3 0
After an array, brackets index from 1. Semicolons separate axes; an empty field selects the whole axis.
v←10 20 30 ⋄ v[3 1] ⍝ 30 10
m←2 3⍴⍳6 ⋄ m[;2] ⍝ 2 5
After a function, brackets qualify axes.
+/[1]2 3⍴⍳6 ⍝ 5 7 9
See axis rules, assignment and Squad.