Downtime is time when you can relax and not worry about work or school. It's usually considered a good thing.
You hear people talking about downtime often, especially when they're planning their day or weekend. They might say things like "I've got some free time on Friday evening" which means they have some downtime to do whatever they want. Sometimes people use it in technical contexts too - for example, computers need regular downtime to update and restart so they can keep working properly. In that case, the idea is the same as with humans: taking a break helps things run smoothly later on.
Time lost due to the failure of some system or machinery, such as a computer crash or power outage.
