SLA stands for ?Service Level Agreement?. Since cloud services are not a product you buy, but rather, an ongoing service, there needs to be some standard as to what quality that service will be.
Just like any good cloud service, Office 365 has a SLA. Typically an SLA is structured on a sliding scale. (IE: if we fail to provide service for 50 minutes in a month then you get 25% money back. If we fail to provide service for 3 hours, then you get 50% back, etc.) The Office 365 SLA functions in the same way. You can find the full details at the link below.
Click here for the full SLA: Microsoft Online Services SLA