This metric measures the amount of memory used in the buffer cache by the largest object (based on the number of pages). It checks the sys.dm_os_buffer_descriptors to identify the object, and returns the relative percentage used. You should use this metric if you want to monitor what is in the buffer area, or if you are having performance-related disk read problems.
Memory is one of the most important resources for SQL Server, so it’s important to make sure SQL Server is using it efficiently. For example, if 90% of the buffer pool (memory area) is being used to store data from one table, it is important to try to optimize the size of this table to save space for other tables in memory. It is very common for one or two objects to be responsible for using a large amount of the buffer cache. To increase the efficiency of the buffer cache area, these objects may benefit from a schema revision (datatype changes or sparse columns), and are great candidates for compression.
For more information, see http://blogs.msdn.com/b/chadboyd/archive/2007/02/02/sys-dm-os-buffer-descriptors-aggregations.aspx and http://www.simple-talk.com/community/blogs/jonathanallen/archive/2012/06/12/110696.aspx.
Note: As with all metrics, you must test this T-SQL script before running it in a production environment. This metric will scan the buffer cache, so if you have a server with a lot of memory, this may take some time to run.