It is just cached data that may be released when another app demands memory.
Here is good description from Investigate memory usage with Windows 7 Resource Monitor:
quote from the link:
The Standby list, which is shown in blue, contains pages that have
been removed from process working sets but are still linked to their
respective working sets. As such, Standby list is essentially a cache.
However, memory pages in the Standby list are prioritized in a range
of 0-7, with 7 being the highest. Essentially, a page related to a
high-priority process will receive a high-priority level in the
For example, processes that are Shareable will be a high priority and
pages associated with these Shareable processes will have the highest
priority in the Standby list.
Now, if a process needs a page that is associated with the process and
that page is now in the Standby list, the memory manager immediately
returns the page to that process' working set. However, all pages on
the Standby list are available for memory allocation requests from any
process. When a process requests additional memory and there is not
enough memory in the Free list, the memory manager checks the page's
priority and will take a page with a low priority from the Standby
list, initialize it, and allocate it to that process.