Im not sure what's your doubt.But I will still try to explain it.
Which is the pressure at any instant (not necessarily equilibrium).It's not a constant.
Which is pressure of CO2 at equilibrium.Its a constant value it tells the value of pressure of CO2 at equilibrium.
At equilibrium Qp=Kp
If Qp>Kp reaction has gone more forward thus it will go in backwards.
And vice versa.
You can define Qc in a similar way as well.