using lets you reference types from namespaces.

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

Example namespace:

namespace MyGame.Player
{
    public class PlayerController : MonoBehaviour
    {
    }
}

Typical Unity script:

using UnityEngine;

public class Player : MonoBehaviour
{
    [SerializeField]
    private float speed = 5f;

    void Start()
    {
    }

    void Update()
    {
    }
}