The "/E" means that the named chord is played over an E in the bass.
The D#b5 is almost a diminished chord, but for a dim. chord to fully qualify as a dim. chord it needs to have four notes (more on this below). Your piano chord had only three, so I identified it ([i]incorrectly[/i], as it turns out) a D#b5 chord. I should have written [b]D#min b5 chord[/b]. And that is:
D# F# A
Here's how this breaks down...
D# minor would be D# F# A#, where the A# is the fifth. Since the A# is flattened by a semitone, it's a D#min b5. And if that chord had a C on top it would have been a full-fledged D dim. chord.
To further the point for identifying the chord as minor-flat-5 as opposed to diminished...
If you play D# F# A and put a B on the bottom, you have a B7 chord ("B dominant 7th"). So the flat-5 triad can go both ways: a dim. chord with a C on top, or a B7 with a B on the bottom.
So while the chord as you used it in your arrangement definitely has a diminished quality, were you to write out a chart for someone you would want to write D#min b5 (as opposed to D# dim) so that they wouldn't play the C. Not that the C would sound bad, but it would lend a different flavor to what you'd written.
Also, one fine "theory point". When you're working in minor, you would write out E G B as (1, 3, 5) because in minor, the third is not "flat" as it's not being compared with anything else, and it's not an enharmonic or altered tone in the scale.
E Major Chord, triad, = E G# B (1, 3, 5)
E Minor Chord, triad, = E G B (1, 3, 5)
Thanks again for your welcome! Looks like I'll be sticking around for a bit... : - )