what does C/D mean on guitar tabs?

Answer 1

C/D in tabs means slide up to D from C.

Why do people who know nothing about chords and chord structure answer these questions!
Anytime you see a chord with a slash (C/E.) it COULD indicate, (as mentioned by others who have attempted to answer your question,) to play the chord indicated by the first letter to the left of the slash (i.e. C,) with the note indicated by the letter to the right of the slash as the bass note (i.e. E) It's called an inversion.

HOWEVER, what the others fail to realize is that with a slash chord the note to the right is a note which is included in the basic harmonization of the Major chord. I - III - V and sometimes the IV is substuited for the III. (i.e. for the C chord that will be C-E-F-G.)

SO, you see there is no D in the basic harmonization of C chord. So their answers can not be the answer you seek.
Should one wish to add the D to the C chord, the D becomes a chord extension and would create the Cadd 9 chord and would be written as such and not as C/D.
FURTHERMORE, what the others failed to realize, and the key to the whole question is the inclusion of the word "tabs."

In tabs there are symbols that represent certain actions to be preformed in relationship to the proper execution of the composition.

Tab Symbols
| = bar line
|| = double bar, end of section
||: = start repeat
:|| = end repeat
% = repeat measure
h = hammer-on
p = pull-off
^ , b, or ( ) = bend
r = bend release
~ or v = vibrato
tr = trill
s = slide
/ = slide up
\ = slide down
T = tap
S = slap
P = pop
TP = tremolo picking
PM = palm muting
[5] = natural harmonic
<5> = artificial harmonic
! = repeat beat (¼,8th)
!! = repeat beat (16th)
u = tremolo bar up
d = tremolo bar down
du = tremolo bar dip
ud = tremolo bar inverted dip
= = hold (bend or tremolo bar)

In conclusion, by looking at the preceding chart you will find the “/” means to “slide up” The answer to your question is, C/D in tabs means to slide up to the D from the C.

Answer 2

If you're talking about a chord, it means play a C chord with a D bass note. In this case you would play an open D string instead of playing the E on the second fret. Or if you are in Drop D tuning, you'd play the low D string.

Answer 3

it means you play a C chord with D in the bass.