x is the variable in the anonymous function 'x => x != ""'. I don't know C# but I presume that everything before '=>' is the argument list and the rest is the function body (with implicit return).

You can see it again in '(a, b) => a + "\n" + b'.

As a side note, I find it a bit funny how C# chooses method names in Linq for the benefit of programmers who know SQL, despite `Select`, `Where`, and `Aggregate` acting exactly like what other functional languages would call `map`, `filter`, and `reduce`. Not wrong, just sort of quirky.

The argument to Where() is a Func<TSource, Boolean>, and that "x => x .. " business is C#'s lambda syntax. x is just a variable name, no magic there.

It's a lambda. Saying x => foo(x) is the same as lambda x: foo(x) from Python.

x is the input variable to that anonymous function.

Instead of the named syntax e.g. doSomething(x) { return x * x; } the syntax for C# anonymous functions is x => x * x