F9 refers to the f-stop that the lens is set at. In the old days we set the f-stop on the lens itself. With your camera in the Aperture Priority Mode there most likely will be a wheel on the camera near the shutter release button for you to change the f-stop.
What an f stop is, is a reference to how big the shutter will open which controls the amount of light that gets in. What confuses most people is that as the f-numbers get bigger (or higher) the opening gets smaller and thus lets in less light. In fact, as you move up one setting it is basically letting in half the amount of light than the previous setting. Conversely, one setting lower than the one you are using lets in twice as much light as the setting before it.
What you should be concerned about with your application is that 1. You are letting enough light in to properly expose the subject and 2. You are using a high enough f-stop to get proper depth of field so the entire subject is in focus. As the f stop number gets higher, letting in less light, the distance of objects in the frame of the picture that will be in focus (acceptable range of sharpness) will increase. That is why I suggested above that you play with various f-stop settings in the Aperture Priority Mode that you have available on the camera instead of the P mode you have been using. This will set the shutter speed automatically for the f-stop you have chosen. This is important because as the lens opening gets smaller, letting in less light, the length of time the shutter must remain open needs to increase to get proper exposure.
Doing the exercise mentioned in the prior post will start to give you a feel for how each different stop changes your finished results.