A set is a collection of distinct items where order does not matter. Think of it like a bag of marbles: you can see which marbles are inside, but there is no "first" or "second" marble.
The set $\{1, 2, 3\}$ equals $\{3, 2, 1\}$ because sets ignore order. Also, $\{1, 1, 2\}$ equals $\{1, 2\}$ because sets have no duplicates.
Sets work differently than arrays. In an array, $[1, 2, 3]$ differs from $[3, 2, 1]$ because position matters. Sets throw away that information.