What is the com.android.email/cache folder? I am looking at where data is stored on an Android device.  The ending of the path is com.androidemail/cache/then a jpg.  What is the email cache?
Cache is a kind of temporary storage for faster access to e.g. files loaded via network. In case of an email app, this most likely holds contents that where attached to a mail (like images: you even named a .jpg, that's it), so it doesn't need to be re-loaded via the network each time the user looks up a message (see: Web cache).

Also see our cache tag-wiki.