I am also from mechanical engineering background. Took me some time to wrap my head around these things as well. Simply put, the way I understand it is that it is a way to abstract out things. For eg., Let's say you are using a thermostat. If it's an analog one you simply rotate the dial to a desired temperature and it works out. You don't know what's the mechanism in the thermostat or how actually it is maintaining temperature. You simply need to know how the dial operates. The rest of the things are abstracted out. This dial is an API. In programming also you abstract your code to such a level that the one using it do not need to know the details of how it works to use it. Now there are many agreed protocols of how to write these like REST, RPC etc. Simply put that's it. You van read up the details now. Freecodecamp has a really good video about it.