535fb36d_Using_ALTER_INDEX__Description
[Setting] MAXDOP [Values] 0 – (max num processors) [Description] 
Indicates to SQL Server how many threads it can use for the operation. 0 defaults to the default, which is equal to the number of processors on the system. Sometimes you will want to limit this to 1 or 2 so it does not strain your disk subsystem.
Using ALTER INDEX
Description