Lemmy is like a state/province made up of several cities. You start by picking a city ("instance") to live in (lemmy.world, lemmy.ml, etc.). You can visit and socialize in a pub (a "community" - like Lemmy.world's c/mildlyinfuriating) located in your own city. Alternatively, you can drive over to socialize in a pub in a neighboring city (lemmy.ml's c/memes). In the neighboring city's pub, you might socialize with people that live in your own city and those from other neighborhoods. If a neighboring city decides they don't like your city, they can tear up the roads between the two of you - this is called "defederation." You can fly your drone over and still see and hear what's going on in this neighboring city's pubs, but you can't socialize with them.
I'm sure this analogy breaks down in several ways, but it's how I have thought of it and explained it.
Grab an Elegoo Arduino kit on Amazon and start there to get started on the coding journey! Follow Paul McWhorter's Arduino tutorials on YouTube. You can program the Arduino from knowing NOTHING about electronics and coding. He teaches you from square one as if you had zero knowledge. I'm on lesson 35 myself and it's addicting! Coding something yourself and watching it work is amazing! I've even used what he taught me to develop a temperature sensor for my house that helps alert me when the pipes underneath my house get too cold during the winter. It's going to help so much and be such a valuable machine.