Jump

Lesson 3 of 20 in Coddy's Unity Game: Sky Lanes course.

Next the runner jumps. A CharacterController has no gravity of its own: your script keeps a vertical speed and moves by it every frame.

Gravity changes that speed every frame. When the runner stands on the ground (controller.isGrounded), keep a small downward speed of -2 so it stays pressed onto the road:

if (controller.isGrounded && fallSpeed < 0f) fallSpeed = -2f;
fallSpeed += gravity * Time.deltaTime;

To reach a height h under gravity g, a jump starts with the speed Mathf.Sqrt(h * -2f * g). With a jump height of 1.6 and a strong gravity of -40 the runner jumps high and comes down fast, which suits a game where things come at you:

bool jump = Input.GetKeyDown(KeyCode.W) || Input.GetKeyDown(KeyCode.UpArrow) || Input.GetKeyDown(KeyCode.Space);
if (jump && controller.isGrounded)
{
    fallSpeed = Mathf.Sqrt(jumpHeight * -2f * gravity);
}

The move now has a y part too: fallSpeed * Time.deltaTime.

Challenge

Easy

Add public float jumpHeight = 1.6f;, public float gravity = -40f; and private float fallSpeed; to Runner.
After the lane keys, read the jump keys (W, Up, Space) and jump only when controller.isGrounded.
Before targetX, add the two gravity lines above.
Make the move new Vector3(x - transform.position.x, fallSpeed * Time.deltaTime, 0f).

Try it yourself

using UnityEngine;

public class Runner : MonoBehaviour
{
    public float laneWidth = 2.5f;
    public float laneSpeed = 16f;

    public int lane;

    private CharacterController controller;

    void Start()
    {
        controller = GetComponent<CharacterController>();
    }

    void Update()
    {
        if (Input.GetKeyDown(KeyCode.A) || Input.GetKeyDown(KeyCode.LeftArrow)) lane = Mathf.Max(-1, lane - 1);
        if (Input.GetKeyDown(KeyCode.D) || Input.GetKeyDown(KeyCode.RightArrow)) lane = Mathf.Min(1, lane + 1);

        float targetX = lane * laneWidth;
        float x = Mathf.MoveTowards(transform.position.x, targetX, laneSpeed * Time.deltaTime);
        Vector3 move = new Vector3(x - transform.position.x, 0f, 0f);
        controller.Move(move);
    }
}