The affinity Input-Output (I/O) mask Server Configuration Option.
To carry out multitasking, Microsoft Windows 2000 and Windows Server 2003 sometimes move process threads among different processors. Although efficient from an operating system point of view, this activity can reduce Microsoft SQL Server performance under heavy system loads, as each processor cache is repeatedly reloaded with data. Assigning processors to specific threads can improve performance under these conditions by eliminating processor reloads; such an association between a thread and a processor is called processor affinity.
References:
http://msdn.microsoft.com/en-us/library/ms189629.aspx