fn x  =  means that if the second argument to
fn is an empty list, return an empty list.
fn x (True:ys) = x : fn x ys means that if the second argument to
fn starts with
True, return a list that starts with the first argument to
fn and continues with
fn x applied to the remainder of the input list.
fn x _ =  means that if the second argument to
fn is anything not covered by the previous cases, return an empty list. (This line means that the first line in the definition of
fn is actually unnecessary.)
fn x takes a list and returns a list made of as many copies of
x as there are consecutive
True values at the start of the input list.