To answer this question, it's probably helpful to answer another: What is a computer? Practically speaking, a computer is a data tool. Computers store vast amounts of data (such as music, photos, email, homework, social media profiles, etc) and allow us to retrieve it later. They also provide safeguards to ensure we interact with our data correctly. Chances are, if you had to (and could) manually upload a photo on instagram without the app, it would take you a lot longer than 20 seconds to do it.
But what is Programming? Programming is feeding instructions to a computer to make it do something. Need to tell your phone to wake you up at 5:30 AM? Or do you wish you had a calculator to do your algebra homework for you? Do you need to launch the missiles? Any time you convince a device to carry out a task for you, you are in effect, programming. Some tasks are more difficult than others, but with enough diligence and creativity, it can probably be done.
So how do we harness this power? It all comes down to data. At the heart of programming, we have two main tools at our disposal: Input and Output. Know something the computer doesn't (like your Gmail username)? Tell the computer what it is (input) and it'll be able to use that. Do you need info from the computer? Maybe a new email came in and you want to read it? Ask it to retrieve that data and it will show you (output). Computers act on input and respond with output.
Wait a second, you might say, setting an alarm clock is not really "programming!" Ah, the principal difference between setting an alarm clock and programming is only in the choice of inputs you are allowed. An alarm clock allows only a few specific inputs: The time it should ring, which days it should ring, alarm volume, which alarm sound should be used, etc. When we write software, the variations of input are much greater, and also much more primitive. We don't deal with alarm times, or days of the week; we deal with numbers, letters, and symbols. This may sound very limiting, but like legos, we can build some very interesting things from some very simplistic parts.
You can follow along on your computer at try.purescript.org (this doesn't work on phones unfortunately). When the website loads, delete everything in the textbox on the left side of the page except the top three lines. Your code page should read the following:
module Main where import Prelude
As you follow along, notice that any time you type anything, there are various symbols appearing on the left. If you move your mouse over these symbols, you'll see different warnings and errors from the compiler. These messages are here to help and guide you.
What's a compiler? It's a tool that translates your code (the text you type) into instructions that can be run by the computer. Writing code can be tough, and sometimes we'll make mistakes; whenever the compiler detects errors in your code, it will give you a description of the error along with which line the error has occurred. It will also show warnings as an ⚠️ symbol. You may safely ignore those for the time being. Don’t feel compelled to type down every codebox in this book, I will inform you of which ones should be typed. However, feel free to experiment as you see fit.