Alignment, Auto Width and Remove This Column: Obvious meanings.
Adjust Column Width: Changes the column width so that the column becomes wide enough to display its contents and caption.
Field Chooser opens a list of fields (columns) from which you can add to the displayed columns by dragging out new fields, or remove displayed columns by dragging their headers into the list. See Adding and Removing Columns.
Format Columns opens the Format Columns dialog, where you can set alignment, specify display format, change their appearance and so on.
Note that not all of the above-mentioned items are available for all panels. For instance, the Display Format menu item only presents in the Report, Details and Disassembler context menus that are called over column headers.