Class = component

your project so far

Still reading. The Player.cs shape here is the shape of every script you write from lesson 06 on.

Start with one word: GameObject. Everything that exists in your game — the floor, the camera, a character, a bullet — is a GameObject.

On its own a GameObject is an empty doll. It has a name and a place to stand, and nothing else: you cannot see it, you cannot touch it, it does nothing.

You make it real by clipping parts onto it. Each part is called a component. And the C# class you are about to write is simply one more part you clip on.

animated diagram

Watch the four parts clip on one by one — only after the last one does the character actually move.

The four parts, in plain words

Transform — where it stands, which way it faces, how big it is. Every GameObject has this one and it can never be removed.
Mesh Renderer — the part that actually draws it on screen. Take this off and the character is still there, just invisible.
Collider — an invisible body. Without it the character walks straight through walls and the floor.
Player.cs — the part you write yourself. It is what decides: when the player presses W, walk forward.

Now write the fourth part yourself

Unity wrote the first three parts for you. The fourth one — the brain — is a C# file. Every Unity script has the same shape, and you only ever change what is inside the braces.

Player.cs — the file name must match the class name

using UnityEngine;                          // gives you Vector3, Debug, ...

public class Player : MonoBehaviour         // ": MonoBehaviour" = attachable component
{
    public float speed = 5f;                // shows in the Inspector
    [SerializeField] private int hp = 100;
    private bool isDead;                    // pure code, no Inspector

    void Start()
    {
        Debug.Log(name + " ready, hp = " + hp);
    }
}

The line ": MonoBehaviour" is the whole trick. It is what turns an ordinary C# class into a part you are allowed to clip onto a GameObject. Leave it out and the script cannot be attached to anything.

The file must be called Player.cs — Unity matches file name to class name or the component will not attach.
public = other scripts can read it AND it appears in the Inspector.
[SerializeField] private = the best default: visible in the Inspector, invisible to other scripts.

animated diagram

A field is declared once in code; after that you tune it in the Inspector while the game is running.

Create and attach a script

1
In the Project window: Right click▸Create▸MonoBehaviour Script (older Unity: C# Script).
2
Type the name immediately — renaming later does not rename the class inside.
3
Select a GameObject in the Hierarchy, then drag the script onto the Inspector.
4
Press Play. Anything you Debug.Log appears in the Console window.

Reaching other components

C# script

private Rigidbody rb;

void Awake()
{
    rb = GetComponent<Rigidbody>();   // on this same object
}