The dyad { (from) is used to select items from an array. This is sometimes called indexing an array.
a =. 5 7 9 4 8
3 { a NB. select item at index 3
4
0 { a NB. indexes start at 0
5
2 4 { a NB. select 2 items
9 8
2 0 4 { a NB. select 3 items
9 5 8
From selects items from its argument, and therefore selects rows from tables.
m =. i. 3 4
m
0 1 2 3
4 5 6 7
8 9 10 11
0 { m
0 1 2 3
2 1 { m
8 9 10 11
4 5 6 7
From selects tables from rank 3 arrays.
1 { i. 2 3 4
12 13 14 15
16 17 18 19
20 21 22 23