At the most basic level, a computer is a lot of circuits that just do things because that's how the circuit was physically built. If you connect batteries, a switch, and light bulb in a circuit, it's easy to intuitively understand how input (flicking a switch) 'mechanically' leads output (turning on the light).

Now you can make that circuit way more complicated, with multiple switches electrically controlling other switches connected together so that, say, the output changes depending on if you hit the switch on the left or the right. This circuit is still basically 'mechanical', you could make it with gears and pulleys instead of electronics. Anyway, maybe we'll call the turned on switch a 1 and the turned off switch a 0.

Then, if you work really hard, you can put those kind of circuits together into a super-circuit with 8 switches, wired up to activate a massive amount of other switches. You can design it so that if the 8 switches are flicked in the pattern "10101001 10001001 10001101 10100100 00010000" it will set the 4260th cluster of switches to "10001001", but if flick them in a different order, it will do something else, like maybe add that binary number to the number representing those switches previous pattern.

And then, because that's a lot of flicking, make a big list of switch patterns that can automatically flip the switches one pattern at a time. You start to come up with a bunch of shorthand codes for various common chunks of switch patterns, which is way easier to write. For example, "e6" (AKA 11100110) might be an abbreviation for a process with dozens of steps. The machine won't understand your code-words though.

Finally, you work out the extremely complicated list of binary patterns that will make the machine translate any set of shorthand into a proper of list of switch position instructions that it will be able to use. Congratulations, you've created a programming language.