One service, many machines
A streaming platform might use different computers for accounts, recommendations, and video delivery. Some of these parts may be microservices. No single computer does everything, but to viewers the parts work as one service.
Spreading work across computers can help a service handle more users, serve people in different places, and keep working if one computer fails.
The network changes the rules
When computers communicate over a network, messages can arrive late, twice, out of order, or not at all. One computer can fail while others keep working. The system needs ways to detect failures, retry safely, and decide when information on different computers must agree.