public void OnTriggerEnter(Collider collider)

Collider

OnTriggerEnter is invoked when two GameObjects with a Collider component touch or overlap, and one of the Collider components has the Collider.isTrigger property enabled. A trigger Collider doesn't register collisions with an incoming Rigidbody and doesn't collide with any other GameObjects that have Colliders on them. The events are invoked during simulation, which happens after all FixedUpdate methods are called, or within the scope of Physics.Simulate, if you're using manual physics simulation. Any Collider on a GameObject that has a Rigidbody component, or on a child of a GameObject with a Rigidbody component can create OnTrigger events. To use the following code sample, create a primitive GameObject, and attach a Collider and Rigidbody component to it. Enable Collider.isTrigger and Rigidbody.isKinematic. This GameObject will travel forward, until it collides with another GameObject. When a collision occurs, the direction immediately reverses.

using UnityEngine;public class Example : MonoBehaviour{    private float speed = 2f;    //Moves this GameObject 2 units a second in the forward direction    void Update()    {        transform.Translate(Vector3.forward * Time.deltaTime * speed);    }    //Upon collision with another GameObject, this GameObject will reverse direction    private void OnTriggerEnter(Collider other)    {        speed = speed * -1;    }}