The tower of abstraction is a way of looking at the complexity of technology. At the base layer, we have the hardware mechanisms depended upon in computation, such as transistors in the CPU and memory cells in RAM. Above that, we have integrated circuits. Above that, you have machine code, assembly, and the operating system. And above that, several layers up, you have the browser, and its JavaScript runtime. Each layer abstracts away complexity so that the layer above can leverage that complexity without too much effort:

When we write JavaScript for the browser, we are already operating on a very tall tower of abstraction. The higher this tower gets, the more precariously it operates. We are dependent on every individual part working as expected. It's a fragile system.

The tower of abstraction is a useful analogy when thinking about our users as well. When we write code, we are adding to this tower, building upon it layer by layer. Our users will always be situated above us on this tower, using the machinery we've crafted to drive their own ends. These users may be other programmers that utilize our code, building yet more layers of abstraction into the system. Alternatively, our users may be the end users of the software, typically sitting atop the tower and leveraging its vast complexity by a simplified GUI.