When SQL Server needs a free memory page (for example, when reading a database page from disk into the buffer cache), and no free pages are immediately available, the connection needing the free page must wait while SQL Server makes buffers available. This results in slower performance. In the worst case, the connection will have to wait while SQL Server writes a modified page out to disk in order to make a free buffer.
The term Foglight for SQL Server uses to describe the amount of space that is available for allocation for a File Group. Free potential is defined as the amount of free space in the file group, plus the file group's Potential Growth (the amount by which the file group's files could grow).