If something happens markedly, the difference is big enough that you notice it without trying. A student can improve markedly; two rooms can look markedly different; a voice can be markedly quieter than before. It is a fairly careful word — essays, news, and reports use it more than chat. Friends more often say “a lot” or “clearly”. It does not mean “with a pen mark on it”. It comes from marked in the sense “easy to see”, as if the change had a sign on it. Though actually, it still grades how strong the change is: markedly is stronger than “a bit”, but it does not have to mean “completely”.