Key Results have different difficulty (see What is Key Result Difficulty?) and that changes what progress qualifies as success. A Moonshot can be considered successful at 30% progress but a Must Hit 100% Key Result should really get to 100%. So success level is a way to show the expected level to achieve for a successful Key Result.
The Objective success level is the weighted average of the success level of its Key Results
The Success Level is shown in the various progress bars and progress charts as a green area above the success level
The values can be changed to match your organisation's preferred labels & levels - see How to change Success Level & Difficulty values.
Notifications are sent out when Key Results reach the Success & Outstanding levels - see When are Success Notifications sent?