An artifact is any object that has been altered or used by humans. The computer you are sitting at is a modern artifact. Archaeologists are interested in much older artifacts, from the prehistoric or historic past. But, they don't study artifacts because they look pretty - most of what they find was garbage when it was dropped or thrown away centuries ago. Archaeologists study artifacts in order to understand and learn about the people who left the artifacts behind.
Archaeologists distinguish between artifacts and site features.
Artifacts are things that are portable, like stone tools, bones, pots, or bottles. Features are traces of humans that can't be moved, like fireplaces and firepits, house foundations, or wells. To an archaeologist, artifacts can tell a great deal about how people lived in the past, but most of this information comes from the context the artifact is found in. For example, a single arrowhead can tell us that people were in the area, but an arrowhead found along side other artifacts and related features can tell us a great deal more about how these people lived. That's why it is so important not to move artifacts when you find them.