It's really hard to explain in words even in a book let alone in a few paragraphs.

Perhaps an analogy:

Langton's Ant is a mathematical curiosity. The idea is an infinite grid. The 'ant' is a marker with position and direction and gets set in motion from anywhere in any direction (the grid is infinite and uniform so it makes no difference). When the ant lands on a cell (a grid square), if the cell is white it turns it black and turns right, if the cell is black it turns it white and turns left. The future behaviour of the ant and the grid is entirely determined by those two rules and the colour-scape of the grid. No other information is present. But the picture it creates is immensely complicated.

Fractals like the Mandelbrot Set and Julia Sets could be another example.

A simple set of iterated rules can produce a very complicated structure and do so repeatedly and reliably. If you don't change the rules you'll always get the same structure.

The degrees of separation between the rules that DNA (combined with all sorts of biochemistry) provide and the physical structures they lead to are as a chasm but they are still pretty reliable and predictable so clones will reliably look almost exactly the same as each other (same rules, same outcome).

Another analogy would be trying to understand how a sequence of zeroes and ones, or the simple rules of machine code, can lead to what we can achieve even just nowadays with AI (outthinking grandmasters at Chess, generating convincing art, etc). It boggles the mind (or should). The rules of DNA (and their interaction with all sorts of biochemistry) are arguably much more complex and varied than machine code so it shouldn't really come as a surprise that it can produce an infinitude of possible biological shapes and yet do so as predictably and consistently as a computer program.

Does that make sense?