PlayerController.cs

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

[System.Serializable]
public class Boundary
{
    public float xMin, xMax, yMin, yMax;
}
public class PlayerController : MonoBehaviour
{
    public float speed;
    public Boundary boundary;

    public GameObject shot;
    public Transform shotSpawn;
    public float fireRate;

    private float nextFire;

   void Update()
    {
        if (Input.GetKeyDown("space") && Time.time > nextFire)
        {
            nextFire = Time.time + fireRate;
            Instantiate(shot, shotSpawn.position, shotSpawn.rotation);
        }
    }

    void FixedUpdate()
    {
        float moveHorizontal = Input.GetAxis("Horizontal");
        float moveVertical = Input.GetAxis("Vertical");

        Rigidbody2D rigidbody = GetComponent<Rigidbody2D>();
        rigidbody.velocity = new Vector2(moveHorizontal, moveVertical) * speed;

        rigidbody.position = new Vector2
            (
            Mathf.Clamp(rigidbody.position.x, boundary.xMin, boundary.xMax),
            Mathf.Clamp(rigidbody.position.y, boundary.yMin, boundary.yMax)
            );

    }

}