Simple. There is a pattern to it.

111000111000111000111000 may look confusing, but it's simple. It has a pattern. The next numbers are predictable.

1230984029384029352763923095720384 is complex. There is no pattern. The next numbers are not predictable.

In structures, a cube cut into 1000 smaller cubes of equal size is simple.

A cube cut into 2389 smaller figures, random in all 3 dimensions is complex.