The maximum number of light bounces. Lowering this value slightly improves performance, increasing it slightly improves accuracy. The change in Corona is however very small compared to other renderers, so keeping the default value is advised.
You can find Max Ray Depth parameter in Render Setup > Performance > Performance Settings. It governs both maximum number GI bounces and maximum number of allowed reflections / refractions. The default value of 25 means that for each ray, a maximum of 25 intersections is allowed - be it diffuse bounces, refractions or reflections. For a solid glass object, one intersection takes place when the ray hits its surface, and second one when it exits the object. Increasing Max Ray Depth to a higher value than default, usually brings no visual benefits.
Examples
1.1. Crystal - MRD = 25 (default value)
1.2. Crystal - MRD = 1
1.3. Crystal - MRD = 2
1.4. Crystal - MRD = 3
1.5. Crystal - MRD = 100
2.1. Two mirrors facing each other, camera between them - MRD = 25 (default value)
2.2. Two mirrors - MRD = 1
2.3. Two mirrors - MRD = 2
2.4. Two mirrors - MRD = 3
2.5. Two mirrors - MRD = 100
3.1. Room - MRD = 25 (default value)
3.2. Room - MRD = 1
3.3. Room - MRD = 2
3.4. Room - MRD = 3
3.5. Room - MRD = 100