You probably know that the is used before a noun in situations where both writer and reader, or speaker and listener, know what is being referred to. That is often the case when the noun has already been mentioned. The use of the in your example is probably best explained in the words of the ‘Cambridge Grammar of English’ by Carter and McCarthy:
The is most commonly used to refer to things which are part of the speakers’ shared world. It is a way of saying ‘You know which x I am
The fundamentals and the basics refer to a category of things that everyone knows about, even though they may not previously have occurred in the text. Other such words are the essentials, the rudiments, the contents, the rules, the orders and no doubt many more.