For example, Em(onD) and Em(onC♯). I found these in a Japanese guitar tab book I bought.
As @NReilingh says, it's probably a Japanese alternative to "slash notation".
"C/B" for example -- often read aloud as "C over B" means a C chord played over a prominent B bass note.
The easy way to play these is to have a bassist! You play a normal C, the bassist plays the B.
Without a bassist, you need to sound the bass note yourself. For example, to play G/F# you might use the 2nd fret of the bottom E string, instead of the 3rd fret you'd normally use.
It's probably a shorthand for slash notation. Try putting the "on" note in the bass below whatever other chord is notated.
Don't let the term "slash notation" bother you. It's simply a chord that has a bass note(the lowest note) that is not normally part of the chord. You find a lot of these chords in songs by The Beatles for instance.
While you are learning and so new at guitar, I suggest using songbooks that have chord diagrams each time you need to play the chord. In time, you won't need any help remembering the chords.
In addition to slim and NReilingh's answers:
Usually when you see a chord, the lowest note played is the root note of the chord. So when you normally see a chord marked as
Normally you play the chord going up. So in the
Musicians like playing around with something called "inversions" of a chord, where you change the ordering of the notes. Normally first inversion is where the second note (which in our example is the third degree of the scale) is played first:
In the same way, we have second inversion, which is where the fifth degree of the scale is played first:
So what the others were saying follows this logic. You should probably play an
Hope this helps!