PlayerMove

using UnityEngine;

public class PlayerMove : MonoBehaviour {

    public float moveSpeed = 6f;
    public float jumpPower = 6f;

    Rigidbody2D rb;
    bool onGround;

    private void Start() {
        rb = GetComponent<Rigidbody2D>();
    }

    private void Update() {
        float input = Input.GetAxis("Horizontal"); // -1 .. 0 .. 1

        if (Input.GetButtonDown("Jump") && onGround) {
            rb.velocity = new Vector2(moveSpeed * input, jumpPower);
            onGround = false;
        } else {
            rb.velocity = new Vector2(moveSpeed * input, rb.velocity.y);
        }
    }

    private void OnCollisionEnter2D(Collision2D collision) {
        onGround = true;
    }
}