Taiob Ali walks us through the concept of memory grants:
DesiredMemory: Memory estimated to fit intermediate results in KB for the chosen degree of parallelism. If the query runs in serial mode, this is the same as SerialDesiredMemory.
(Amount needed to store all temporary rows in memory. This depends on the cardinality estimate, expected number rows and average size of row). This is called additional because a query can survive lack of such memory by storing part of temporary rows on hard disk. A query is not guaranteed to have the full amount if the total exceeds the preset limit.)
Read on for explanations of each of the elements in MemoryGrantInfo.