What is a drop?Edit
Drops are items that transfer to your inventory when you kill a monster. Most, if not all, monsters have some kind of drop that you can get. However, they may not drop an item every time. So if you're hunting a certain monster type in search of one drop in particular, it could actually take you a while to accomplish.
The best thing to do in that case, is to turn on your Remote Control, and set it to "Auto Fight". In addition to that, if you continue to get a drop that you find useless, you can drag one of those items from your inventory into one of the available slots at the bottom of your Remote Control, and put a check mark in the "Auto Discard" box. With that checked, anytime you pick up one of those items, you're Remote Control will automatically drop that item for you.
Although all drops have some use in the game, not all will be useful for you until later on. For example, you may pick up some Black Clay but have no current use for it. Later on, however, you need 4 of those (along with other items), to produce a Melting Furnace which in turn is used to produce even higher level/quality equipment.