Question:

What is uptime?

Answer:

Uptime is literally what it means - it is the amount of time your site is online throughout the entire month.

The best uptime would be 100%, but because things can break, many hosts offer 99.9% uptime guarantees. This means that if your site is not accessible for more than 43 minutes in a single month, they will reimburse you, depending on their terms of service.