What does A mean on a camera?

Aperture priority. You choose the aperture and the camera chooses a matching shutter speed. Use it when background blur is what you care about most.

Should beginners use manual mode?

Not usually. Aperture priority gives you the creative control that matters while the camera keeps brightness correct. Manual is for when the light is constant or the camera keeps guessing wrong.

What is the difference between P and A?

In P the camera chooses both aperture and shutter speed. In A you choose the aperture yourself, which decides how much of the scene is in focus.

What do P, A, S and M mean on a camera?

P, A, S, M. Every camera has them and nobody explains them. They are not beginner-to-expert. They are four ways to split one job.

There are only three numbers

A photo's brightness comes from exactly three things: aperture (how wide the lens opens), shutter speed (how long the sensor collects light), and ISO (how strongly that light is amplified).

Set all three and the brightness is decided. The four letters just say who sets what.

The four modes

P — Program. The camera picks aperture and shutter. You still control everything else: focus, white balance, exposure compensation. Use it when the moment matters more than the settings.
A — Aperture priority. ★You pick the aperture; the camera picks a shutter speed to match. Use it when background blur is what you care about. This is the mode most photographers live in.
S — Shutter priority. You pick the shutter speed; the camera picks the aperture. Use it when motion is what you care about — freezing a running dog, or letting a waterfall streak.
M — Manual. You pick both. Use it when the light is not going to change and you want every frame identical, or when the camera keeps guessing wrong.

A and S are not half-measures. You choose the one thing the picture is about, and hand the arithmetic to the camera.

Which one should you actually use?

Start in A. Most photographs are decided by how much of the scene is sharp, and A is the mode that puts that in your hands while the camera keeps the brightness correct.

Switch to S the moment something is moving and the movement is the point. Switch to M when you notice the camera changing its mind between shots that should look the same.

ISO sits underneath all four

In any of the modes, ISO can be automatic or fixed. Leave it automatic while you are learning, with an upper limit so it does not climb into heavy noise. Fix it when you want to know exactly how clean the file will be.

Lensmera puts the same four letters on the same dial a real camera uses, and shows you which number the camera just chose for you. When you eventually pick up a mirrorless body, the dial is already familiar.