I don't understand

using System;
using System.Collections.Generic;
using System.Linq;
using UnityEngine;

class CentipedeScript : MonoBehaviour
{
    public Sprite headSprite;
    public Sprite bodySprite;
    public GameObject mushroom;

    private float height;
    private float width;
    private float headCentipedeRadius = 0.5f;
    private bool leftDirection = false;
    private bool downDirection;
    private bool isHead = false;
    private float speed;
    private GameObject target;
    private bool isDead = false;
    private Vector2 previousPosition;
    private Vector2 moveDownToThisPoint;
    private Rigidbody2D rigidbody;

    void Start()
    {
        rigidbody = GetComponent<Rigidbody2D>();
        if (isHead) GetComponent<SpriteRenderer>().sprite = headSprite;
        else GetComponent<SpriteRenderer>().sprite = bodySprite;

        height = 2f * Camera.main.orthographicSize;
        width = height * Camera.main.aspect;
    }

    void Update()
    {
        if (isHead && (transform.position.x - headCentipedeRadius <= -width / 2 || transform.position.x + headCentipedeRadius >= width / 2) && !downDirection)
        {
            ChangeDirection();
        }
        if (isHead) MoveHead();
        else MoveBody();
    }

    void MoveHead()
    {
        if (downDirection)
        {
            if ((Vector2)transform.position == previousPosition)
                CentipedeStuck();
            previousPosition = new Vector2(transform.position.x, transform.position.y);
            MoveDown();
        }
        else
        {
            moveDownToThisPoint = new Vector2(transform.position.x, transform.position.y - 1);
            MoveLeftOrRight();
        }
    }

    void CentipedeStuck()
    {
        downDirection = false;
        MoveLeftOrRight();
    }

    void MoveDown()
    {
        if(transform.position.y > moveDownToThisPoint.y)
        {
            rigidbody.velocity = Vector2.down * speed;
            transform.rotation = Quaternion.AngleAxis(GetAngleRotation(rigidbody.velocity.normalized), Vector3.forward);
        }
        else if (transform.position.y <= moveDownToThisPoint.y)
        {
            leftDirection = !leftDirection;
            downDirection = false;
            MoveLeftOrRight();
        }
    }

    void MoveLeftOrRight()
    {
        if (leftDirection)
            rigidbody.velocity = Vector2.left * speed;
        else
            rigidbody.velocity = Vector2.right * speed;
        transform.rotation = Quaternion.AngleAxis(GetAngleRotation(rigidbody.velocity.normalized), Vector3.forward);
    }

    void ChangeDirection()
    {
        downDirection = true;
    }

    //Part for Body Moving
    void MoveBody()
    {

    }

    private void OnTriggerEnter2D(Collider2D other)
    {
        if (other.tag == "Mushroom" && isHead && !downDirection)
        {
            Debug.Log("MushroomCollision");
            ChangeDirection();
        }
        if (other.tag == "Shot")
        {
            Destroy(other.gameObject);
            Destroy(gameObject);
            Instantiate(mushroom, transform.position, Quaternion.identity);
        }
    }

    float GetAngleRotation(Vector2 direction)
    {
        return Mathf.Atan2(direction.y, direction.x) * Mathf.Rad2Deg - 90;
    }

    public void SetHead()
    {
        isHead = true;
    }

    public void SetCentipedeSpeed(float speed)
    {
        this.speed = speed;
    }

    public void SetTarget(GameObject target)
    {
        this.target = target;
    }
}