This diagram shows an ideal case where the atmospheric pressure is lower than normal along the top of the map and the pressure is at a higher than normal pressure along the bottom.
At time zero, a thermal shoves air aloft at A and injects a bubble of air into the level of the map. Initially it is at rest with respect to the compass directions. Assume, for the moment, that the other winds don't just carry it along, but that it moves because it is pushed by the pressure gradient (the Red Arrow at A.) Sensing the pressure difference, it begins to move directly to the Low Pressure side.
As soon as it moves in the horizontal, the Coriolis force (the Blue Arrow) starts to push it to its right. So the Coriolis force needs to be added to the pressure gradient force vectorially to find the force on the bubble. The result of the vector additions are the solid Black arrows. A little while later, the parcel will have sped up and arrive at point C. It is now moving faster than it was at B.
The Coriolis force, which is dependent on the speed of the wind, will be pulling to the right more and the force diagram will be that shown at point D. When the parcel gets going as fast as it is going to go, the Coriolis force will just balance the pressure gradient force as at point E. The little Green arrow is just to indicate the motion of the parcel. The air is moving right along there. If it to slow it down, perhaps through friction, the Coriolis force will decrease and the parcel will edge across the isobars towards the Low pressure side.
OK, I'm convinced, would you summarize please?
Back to start.