Methods are where you define the instructions you want your script to execute.

The description in the above link is slightly misleading regarding development in Unity because while the Unity Engine itself does presumably use a Main method internally as the main entry point for the application as described, your scripts will instead use various MonoBehaviour Messages which are methods you can define that Unity will execute in response to certain events.

Also note that the first two sentences on Unity's MonoBehaviour documentation page as of this writing are false ("MonoBehaviour is the base class from which every Unity script derives. When you use C#, you must explicitly derive from MonoBehaviour."). Scripts that you want to attach to a GameObject in the scene do need to Inherit from MonoBehaviour, but there are lots of situations where you do not need to do so.

Calling

Executing a method is referred to as "calling" it.

Methods are executed sequentially from the top down one step at a time. Calling another method will execute it entirely before continuing to execute the calling method. For example:

using UnityEngine;

public class Example : MonoBehaviour
{
    // A Field.
    private int _Counter;

    // _Counter starts at 0.

    // MonoBehaviour Messages do not need to be public for Unity to call them.
    // void is the Return Type.
    private void Awake()
    {
        // The ++ Operator increases the value by 1.
        _Counter++;
        // _Counter is now 1.

        // Call the AddSix method (see below).
        AddSix();
        // _Counter is now 7.

        _Counter++;
        // _Counter is now 8.
    }

    private void AddSix()
    {
        // _Counter is now 1.

        // The += Operator is the same as writing "x = x + y".
        _Counter += 6;
        
        // _Counter is now 7.
    }
}