The partial pressure of a gas is an idea (rather than something we can measure). We think of the total pressure of a mixture as being composed of the pressure of each gas in the mixture. The diagram below illustrates this with three different gases (show as gases A, B, and C). When by itself in a container, each gas has a specific pressure. Those are at the top and labeled as such. Then those same amounts are all three pushed into the same container again, so now the total number of moles has increased and therefore the total pressure.
So even in the mixture, the pressure from a particular gas only results from the particles of that particular gas. So the pressure imparted by only gas A is equal to the number of moles of only gas A times the absolute temperature times the gas constant divided by the volume.
\[P_A={{n_A RT}\over{V}}\]
This is the partial pressure \(P_A\) for gas "\(A\)" which has \(n_A\) moles. It is important to note this is a concept rather than something we can measure directly. The pressure results from the collisions of all the gas molecules with the walls of the gas container. We cannot construct materials that feels a force from some molecules but not others. However, partial pressure is still an very important concept in chemistry. In gas equilibria, we will show how we will measure "concentration" since the partial pressure is proportional to the number of moles of that particular gas per unit volume.