exchange 2010 defrag&white spice

Hi everyone,

i want to know  what the white spice and I want to know when you need the database to defragment?

i ran the blow command in my server i got the below result i need to understand this result please:

[PS] C:\>Get-MailboxDatabase -Status | select name,AvailableNewMailboxSpace

Name                                                        AvailableNewMailboxSpace
----                                                        ------------------------
QAQCValidation                                              8.216 GB (8,821,997,568 bytes)
EMPLOYEE-DB                                              12.11 GB (12,999,852,032 bytes)
NewDatabase                                                 74.81 MB (78,446,592 bytes)
A2015DB                                                   3.324 GB (3,568,926,720 bytes)

thanks in advance