Macros are a very common way to do metaprogramming in Elixir. There are many resources that explain what macros are and how to use them (much better than I could): there's the Macro chapter from the "Getting Started" guide on Elixir's website, an awesome series of articles by Saša Jurić, and even a book (Metaprogramming Elixir) by Chris McCord. In this article, I'll assume you are familiar with macros and how they work and I'll talk about another use case of macros that is rarely examined: doing compile-time things in macros.
Elixir is frequently used in network-aware applications because of the core design of Erlang and the Erlang VM. In this context, there's often the need to connect to external services through the network: for example, a classic web application could connect to a relational database and a key-value store, while an application that runs on embedded systems could connect to other nodes on the network.
Lexical analysis (tokenizing) and parsing are very important concepts in computer science and programming. There is a lot of theory behind these concepts, but I won't be talking about any of that here because, well, it's a lot. Also, I feel like approaching these topics in a "scientific" way makes them look a bit scary; however, using them in practice turns out to be pretty straightforward. If you want to know more about the theory, head over to Wikipedia (lexical analysis and parsing) or read the amazing dragon book (which I recommend to all programmers, it's fantastic).