A DSL is a mini "language" embedded in a Rust macro. It is completely valid Rust because the macro system expands into normal Rust constructs, but it looks like a small language. This allows you to define concise or intuitive syntax for some special functionality (within bounds).

Suppose that I want to define a little calculator API. I would like to supply an expression an have the output printed to console.

XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

Output:

1 + 2 = 3
(1 + 2) * (3 / 4) = 0

This was a very simple example, but much more complex interfaces have been developed, such as lazy_static or clap.