The type of an array is [T; N] where T is the type each element in the array is, and N is the length of the array.
You can use a similar syntax to initialize an array conveniently. [x; y] creates an array with y elements, each of which is initialized to x. That's what's happening in your example.
let array: [usize; 2] = [1; 2];
assert_eq!(array, [1, 1]);
Note that using this syntax to create an array requires that T implements Copy so you can't use it on a Box, for example.
However the same syntax also works with the vec! macro and you can use any Clone type there. So Box does work there, which can be a bit confusing.