Combats you usually engage on RPGs when you walk on the world map or a dungeon.
Usually annoying, but also a way to become godlike in the game by getting experience, money and items.
Yes, no one know why these random animals are carrying 148 gil and a magic fire ring, it will forever be a mistery.... (legendary frog)
In some rpg's, this type of battle can be seen by the characters being place in a battle out of nowhere. Usually, they occur when walking around on the world map, in a forest, or threw a field.
Sometimes, random battles are ok. But when they warp you every 5 seconds, it can be very annoying.