What does the measurement 6.5 v represent?

I'm not sure.
Where did you see it ?

If you saw it on a voltmeter, then it stands for "6.5 volts".
It tells you that the 'potential difference' or 'electromotive force'
between the two points connected to the meter is 6.5 volts.

-- If a 1-ohm resistor were connected between those two points,
then 6.5 coulombs of charge would move through the resistor every
second.  We would say that the "current" through the resistor is
6.5 Amperes, or "six'na half amps".

-- Each coulomb of charge (6.25 x 10¹⁸ electrons) that moves from
one of those points to the other one either gains or loses 6.5 joules
of energy, depending on which direction it moves.

-- So the resistor would need to shed heat somehow, at the rate of
6.5 watts (6.5 joules per second).  If it couldn't do that, then it would
go through an interesting series of changes.  It would get warm, then
get hot, then glow cherry red, then glow bright red, then glow orange,
then glow bright yellow, then make a sound like a "POP".  Then it would
break in the middle, and a little wisp of smoke would come out of it.
The two halves of the resistor would sink, sag from their wires, and
the current between the two points would stop. 

We would say that the resistor had "fused", "failed", "melted", "burned out",
or "bought the farm" ... all because it couldn't get rid of heat fast enough.