A distributed system is a service made from multiple computers that communicate and work together. A website might use separate computers for its pages, saved information, and background tasks.
Think of a team working from different offices: each person has a part to do, and they share updates so the group can finish the job.
If one computer loses connection or stops working, the others may not immediately know. Distributed systems need ways to handle delays and failures, because the parts do not all share one perfect view of what is happening.