The Cargo.toml file is where metadata about the program is stored. That includes the program's name, version number, and authors, but importantly it also has a section for dependencies. Editing the content of the [dependencies] section is how we tell Rust that our code should be linked to external libraries when it is compiled, which libraries and versions to use, and where to find them. External libraries are collections of source code that were packaged up in order to make them easy to use as components of other programs. By finding and linking good libraries, we can save the time and effort of writing our whole program ourselves. Instead, we can write only the part that nobody else has already done.