The top region is the font being edited.
Blue boxes are undefined glyphs.
The bottom region is where you edit the current font. The line of pink boxes in the middle is hold space. Glyphs that are in progress can be put here temporarily. This is also used to automatically save the old glyph when a character is updated, and to save the work area if a new character is selected for editing without saving changes. Glyphs can be dragged from the font to the hold or work areas, or to another character in the font.