That is a measurement of your brake pad thickness,recomended
replacement time is 3mm,if if you have 4mm of pad life left you have
7mm of pad. the rear pads wear at about half the rate of the front.

if you have 13k miles and you have 7mm left on the front you will
likely be replacing pads front and rear at about 25k.

Makhan, as TNKEV said, this is a measure of how much life
you have left on your Accord's "brake linings," where "brake
linings" refer to brake pads for the front brakes, and,
depending on the car, either brake pads or brake shoes on
the rear brakes. Rotors are not involved here at this point,
at least not as indicated on the invoice you have.

The minimum required thickness varies from one model to
another. For example, for the 1991 Civic, 3 mm minimum is
required for the front. For the 1995 Civic, only 1.5 mm is
required for the front.

It's not clear how your service people measured, so I won't
agree with all of TNKEV's post. The dealer might have meant
the pad thickness on the front was 7 mm, meaning, if the min
required is 3 mm, you have 4 mm to go.

To give further perspective: Your front brake pads probably
started with about 11 mm of pad. If your rear brakes use
shoes, the shoes probably started at about 8 mm thickness.
I'd guesstimate you're probably around the one-third to
halfway point in the life of these linings. To say more
exactly, I'd need to know the min thickness specs for your
Accord and the actual starting thicknesses, as well as
whether your car uses pads or shoes in the rear.

The better news is that brake pads that are very near the
end of their service life have a built-in "alarm" to tell
the car driver. There's a piece of metal on the pad that,
with enough wear, will start screeching against the rotor.
It's noisy enough to be annoying, but sounds early enough so
that it's still safe to drive the car another, say, 100
miles.

You may get around 25k miles of life, altogether, out of
your brake linings, the way you're going. That's a bit low,
but it's all relative. Brake lining wear depends on driving
habits. E.g. do you drive a lot of hills or in the city a
lot?

This a manual or auto transmission? If manual, do you brake
with the engine?

Having brake pads and brake shoes replaced is routine
maintenance on a car, to be performed around every 25k to
70k miles.

Use only genuine Honda brake pads and shoes. Using others
may get you more brake lining life, but will tend to shorten
rotor life. (OTOH others here feel that rotors are so cheap
it's not worth worrying about, etc.)