This dialog is used to change the font settings. Click on one of the "Change Font" buttons, to get a font chooser.
The default settings use Courier (non-serif, monospaced) fonts for code and attachment contents, and variable spaced font with serifs for menus, buttons and labels.
True Type Fonts under Unix
If you have TrueType fonts installed, expecco will use those much better looking fonts. Otherwise, the standard (somewhat pixly) X fonts are used.
Notice that TrueType font rendering requires a much higher network bandwidth, and will make the rendering very slow, if you are using a remote X-display server over a WAN connection. In this case, it may make sense to disable Xft fonts, and use the faster (but ugly) X standard fonts.