Simple rules can produce remarkably complex results, basically.

Many fish, for example, have a pressure sensing organ along their sides called a Lateral Line. If the fish next to them changes direction, they feel it, and can echo it. So if one fish in a school spooks, say, or spots a really tasty-looking piece of food, and decides to change direction, the ones near it feel the change and echo it to avoid collision, and others move to avoid them, and so on. Within a fraction of a second the whole school has echoed it.

Army ants are another excellent example. they have very few brain cell, yet they can join together to form bridges to span quite wide gaps using only very simple rules. If they hit a gap, they slow/stop; and as long as other ants are walking over them, they freeze in place. So basically the first ants to hit a gap stop, other ants clamber over them and stop a little further out, and together they become the units of a bridge that others cross. And once the gap is crossed, they effectively reverse the process, climb out of the gap and tag on at the end. It looks like incredibly complex behaviour, yet invidual ants have almost no instructions to follow.