What do "position.transform" and "Input.GetAxis" mean in Unity?

I am making the transition from Game Maker to Unity, but I feel lost when I look at Unity's programming.

When I followed a tutorial to make basic movements, I had this code:

public Vector3 playerPos;
void Update ()
{

    float V_X = transform.position.x;
    V_X += Input.GetAxis("Horizontal") / 2;
    playerPos = new Vector3(V_X, 0, 0);
    transform.position = playerPos;
}

A few things are confusing me here:

• 
  

  What is transform? And why is position something that is a part of it? Why not have the position of the objects represented by just x rather than transform.position.x?

  
  
  


• 
  

  Is GetAxis(...) a command that has a return value? Why is in "sub-categorized" under Input? And why not detect the key presses directly?