As jj has said, the / is used to denote bass notes.
Ebmin/Bb - Cb major (D#min/A# - B major)
first it's giving you what the actual chord change is, so Ebmin/Bb is your i/5, and Cb major is your bVI.
The bracketed notes are what's called an enharmonic spelling. Eb and D# are the same note, they're just named differently depending on what the function of the note is. Bb and A# are also the same note, as are Cb and B.
The reason it says Cb instead of B here is that the function of it is as a flat 6th(bVI), and so it is enharmonically spelled as Cb to make clear it's function.
and the "-" used to separate keys
It's separating chords rather than keys, the - means 'to' in this case, so CMaj - GMaj would mean C major to G major.
I'm completely unaware of what "i/5 - bVI" means.
i means the chord 1, the fact that it's lower case likely means it's a minor chord.
/5 means that the tone that would usually be a 5th above the key note is being used in the bass. This is called a Second inversion which you can read about at https://en.wikipedia.org/wiki/Inversion_(music).
often with this type of notation lower case letters denote minor chords and upper case letters denote major chords, so the i means minor on 1 or - a minor chord with it's root on the first degree of the scale.
and VI means Major on 6.(we'll come to the b in a moment)
I was confused at first seeing the b in bVI, because you can also notate inverted chords in teh classical system using letters, where I would mean a straight root chord, Ia would mean 1st inversion and Ib would mean second inversion. (this doesn't make sense here though because we've already seen the /5 system being used)
so what the bVI means is a major chord starting from the flattened 6th degree of the scale.
Analysis of the progression
I could be mistaken here, chord analysis can get messy! But, I'll give it a go.
based on your bass notes, your 5 in i/5 moves up chromatically to the bVI, which could make some nice voice leading, but aside from that let's look at the chords we get based on C.
i/5 = G C Eb (basic c minor is C Eb G)
bVI = Ab C Eb
or in your case
Ebmin/Bb = i/5 = Bb Eb Gb (basic Eb minor is Eb Gb Bb)
Cb major = bVI = Cb Eb Gb
not only does the bass rise a semitone, but the other 2 notes do not change, so you should come up with a really interesting sound without being over dissonant. Your ear hangs on to the notes that don't change so the ones that do will sound good against them.
Often when chords are re-arranged this way it's because it makes the transition of individual notes smoother, which can sound pretty fantastic. It's called voice leading and you can find a ton of similar patterns in books like Chord Chemistry by Ted Greene if you're a guitarist.
Hope that helps! for all my explanation your ears are the real judges of chords
