There is an explanation on the blog of their Code Analysis Team.
The actual formula for it is:
Maintainability Index = MAX(0,(171 - 5.2 * ln(Halstead Volume)
- 0.23 * (Cyclomatic Complexity)
- 16.2 * ln(Lines of Code))*100 / 171)
But don't just listen to me, go get the full read there.