In C#, higher-order functions refer to functions that can accept other functions as arguments and/or return functions as results. In other words, these functions treat functions as first-class citizens, just like any other data type. This concept is an integral part of functional programming, which aims to treat computation as the evaluation of mathematical functions and avoid changing-state and mutable data.
Higher-order functions enable a more modular and expressive way of writing code, as they allow you to encapsulate behavior in functions and pass that behavior around as needed. This can lead to more concise, readable, and reusable code.