what is "- >" in C# mean

I think you mean '=>'.
It's used for Lambda expressions
Here's a quote from MSDN:
"A lambda expression is an anonymous function that can contain expressions and statements, and can be used to create delegates or expression tree types."
Might I recommend:
Exploring Lambda Expression in C#[^]
and
Lambda @ MSDN[^]

That's the dereference operator (->), which works very similar to the same operator in C++. It is used to access member and methods of a class or structure over a pointer. In C# you can use pointers in unsafe code only, so under normal conditions you will not encounter pointers and the need to dereference them too often.