`Do[expr, {iter}]` iterates `expr` for each value of the iterator.
Do[expr, n]
Do[expr, {i, imax}]
Do[expr, {i, imin, imax}]
Do[expr, {i, imin, imax, di}]
Do[expr, {i, {i1, i2, …}}]
Do[expr, {i, imin, imax}, {j, jmin, jmax}, …]
Do[Print[k], {k, 1, 3}]
→ 1
2
3
Null