Use this option to help quickly set a filter. The value of the cell you click on will be used to set the filter. For example, right mouse click in the Terrain column to set a filter of all records that have that Terrain rating.
Add to existing - A filter with this value will be added to any existing filter (That is, the two filters will be "anded")
Replace - Any existing filter is replaced
Copy to Mfilter (Where) Box - The filter is not actually set, but rather the Sqlite "Where" syntax is copied to the Mfilter box at the top of the screen. This gives you the opportunity to further manipulate the "Where" syntax.
Copy to Clipboard - The filter is not actually set, but rather the Sqlite "Where" syntax is copied to Windows Clipboard.
Note: For the more advanced user GSAK provides a way for you to fully customize this menu and add more options. For more information see this link