The .NET Twilio Server Library (TSL.NET) is an open-source project that simplifies working with the Twilio APIs for .NET developers. It covers both major use cases, including the REST API and TwiML responses, making it easier to send text messages and initiate calls using just a few lines of code. The library also supports generating dynamic TwiML from code, allowing developers to build their voice and SMS-based applications or integrate these features into existing .NET applications.