One of the most important things a hosting provider can provide is good uptime. To stress this to users, they typically offer an uptime guarantee.
A2 Hosting offers a 99.9% uptime guarantee, which permits roughly 42 minutes of downtime per month. If they fail to adhere to this uptime, they will pay you 5% of the monthly fee for every hour your website is offline.
I recommend that you ignore such guarantees and choose a host with historically good uptime (which A2 Hosting has), and which has great support, so when problems do occur they are dealt with swiftly. You can read why here.