It is a testing framework based on the very popular NUnit test libraries. If you have used NUnit in your C# projects before for unit testing, it will be really helpful to understand Xamarin.UITest faster. But if you don't have prior experience in NUnit, it's absolutely fine.
Xamarin.UITest is basically a set of libraries for C# (similar to JUnit for Java) to help write unit tests.
Xamarin also supports the Calabash framework for writing tests, if you want to write them in Ruby and Cucumber.
We'll be focusing on Xamarin.UITest to write tests in C# for continuous testing.