A sugar break is when the specific gravity of a fermenting must reaches a point relative to the original gravity of the must. So for a must that has a original gravity of 1.100 that is expected to ferment dry (1.000), the 1/3 sugar break would be 1.067, the 1/2 sugar break would be 1.050, and so on. If the final gravity is expected to be higher (1.010 or 1.020), then the calculation should be adjusted.
A simple formula to figure out the SG of a given sugar break
SG = 1 + (OG - FG) * SB
where OG is the original gravity, FG is the expected final gravity, and SB is the sugar break you're looking for (1/3, 2/3, etc)
If the gravity stuff makes no sense, do a forum search for hydrometer.
Hopefully that covers it.