Xamarin.UITest is a testing framework based on a popular test library in C#, NUnit. If you have used NUnit in your C# projects previously 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.
It is basically a set of libraries for C# (similar to JUnit for Java) to help write unit tests.
And by using UITest, we will be writing UI tests for our mobile application.