x => method(x) is syntax sugar for a Func instance that takes a variable (x) and runs method on it.
x => method(x)
Func
method
Example 6. LINQ Query Comprehension[7]
from i in a from j in b from k in c select foo(i, j, k);
Example 7. Without the syntax sugar
a.SelectMany(i => b.SelectMany(j => c.Select(k => foo(i, j, k))));
[7] from, in, and select are keywords.
from
in
select