A stroke is a thick or thin outline that is applied on a layer, this option is available on most decent to high standard editing programs such as Photoshop, GIMP, or other editing softwares. A drop shadow is basically a shadow which is applied to give a layer more depth and realism, or can just be manipulated into a block shadow depending on what you're creating. Was that clear and concise or was that too complicated? If you still don't understand go to youtube and search up "how to apply a stroke in photoshop" or something similar to that. :3