I'll explain it from a higher level of abstraction so hopefully you can actually understand what you're doing here. You have an array called highscore. It is important to know that this is an array, and not a list, set, collection, etc. In python (and many other languages) you can use indexing to access data within the array. As others have mentioned, arrays start counting at 0.
So in the first line of code, you're calling the print function to print whatever data is in the 4th 'slot' in the highscores array. We don't know the contents of the array, so it could be numbers, strings, objects, or a mixture of data. So the next line could do a few different things. If the data in the first and fifth slots are numbers, the result of the substraction will be printed. If they are of different types, and an operation is not defined for substraction you will get a TypeError.
Edit:: It's also a good idea to familiarize yourself with the different data structures (Array, List, Set) as they appear in many languages, and their operations are usually very similar / the same across languages.