The 'En' name given to this locus stands for 'English pattern,' which refers to English spotting (aka 'broken' pattern).
Solid pattern is the normal pattern in rabbits, lacking any spots from the En gene. When breeding a solid non-spotted rabbit with one copy of the spotted gene, you will get some with spots and some without (more solids). This is why this little blue fellow in the picture exists, he is a true blue spotless English Spot!
Solid Pattern Colors
Broken pattern rabbits are white with spots. Think of the white parts as a sheet with 'holes' cut into it where the rabbit's real color shows from under the sheet.
The En gene is responsible for broken pattern as well as the famed English Spot pattern (which is selectively bred to have a stripe down the back and specificly placed spots). It is also the same gene behind Rhinelanders, Checkered Giants, Dwarf Papillons, and 'Dalmatian' coloring. These are all just different ways a spotted pattern can be selectively bred.
Spotted Pattern Colors
Charlie pattern occur with two copies of the spotting gene. So instead of one sheet, it's like having two white sheets (resulting in fewer visible spots because the sheets overlapped each other's spot patterns for greater coverage).
When bred to solid (enen, or 'spotless' rabbits), a charlie x solid pairing will produce 100% broken pattern offspring. Because broken pattern (one copy of En) is desired for showing, it's common for breeders to do this pairing.
Charlie Pattern Colors