Are you using a laptop?
To answer your question : Standby memory is memory that the Windows memory manager has moved out from a process working set - but has kept it accessible. The standby memory, or more accurately the standby page list is a sort of cache for application code + data. When an application requires pages that are not in its working set, if these pages are in the standby list, a "soft" fault occurs (as opposed to a hard fault where the pages come from the paging file, the executable on disk, or a mapped file on disk).
Free memory (or the free page list) is memory that has been freed for use and is waiting to enter the zero page list to be zeroed out and given to applications that require pages of memory. Standby memory is able to be used by all applications - but they do not control it, neither do you - only the Windows memory manager governs the usage of the standby page list.