Question

(see the pic) what are the numbers written after F representing? and why are the digits in the opposite order?

Answer 1

F12 represents force of particle 1 on 2.
F21 represents force of particle 2 on 1.

Answer 2

these are representing the attractive forces
they are in opposite order because they are attracting the other body and the attraction is acting towards each other center

let the first body be A and second one be B.
A is exerting a force on the centre of B and trying to pull it towards itself.
Similarly B is also exerting a force on center of A and trying to pull it towards itself.Therefore the arrows show the directions of attractive forces exerted on the bodies and hence are opposite..

hope it helps