This dialog may look different every few times you upgrade eJourn. It's likely that you'll never see it with exactly these options (one of the options isn't even supposed to be there, it's in development).
All of these options are actually plugins (in $PREFIX/share/ejourn/modules, yes I'm aware they should be in lib, and they'll get moved eventually).
Each plugin should be largely self explanatory, and I'll try to cover them all in this manual in the plugins section.
Basically, if it's checked, it's loaded, if not it's not loaded. You can load and unload plugins as much as you want without fear of repercussions. Plugins are all designed to be fully loadable and removeable without program restarts.