The left-hand side of the arrow is a pattern expression [1] (though usually you'd use the :> arrow to delay evaluation of the right-hand side). The "full" notation is x_H for a pattern variable x that needs to have head H. Dropping the H, it matches anything. Dropping the x, it just doesn't set a variable when matching. Dropping both, it matches anything and doesn't set a variable.

[1] https://reference.wolfram.com/language/guide/Patterns.html