Something casual is ordinary or everyday — not too special, unusual, fancy, serious, or eventful. You might have a casual conversation with your classmates before homeroom, or attend a casual backyard barbecue.
If you're going to your friend's house for a dinner of pizza and soda, you could describe the evening as a casual night at home. Formal attire might mean tuxedos and evening gowns, but casual clothing is the kind of clothing you wear every day, for ordinary activities. If you treat a person or a subject in a casual way, you're not paying close attention or treating them as special. And you might remind your siblings to act casual on the day of your mom's surprise birthday party so that they don't ruin the surprise.