MonoBehaviour.OnEnable()

Description

This function is called when the object becomes enabled and active.

Because this function is executed when the object is enabled, it will be executed whenever entering the Play Mode (with the object enabled).

// Implement OnDisable and OnEnable script functions.
// These functions will be called when the attached GameObject
// is enabled.
// This example also supports the Editor. The Update function
// will be called, for example, when the position of the
// GameObject is changed.

using UnityEngine;

[ExecuteInEditMode]
public class PrintOnOff : MonoBehaviour
{
    void OnDisable()
    {
        Debug.Log("PrintOnDisable: script was disabled");
    }

    void OnEnable()
    {
        Debug.Log("PrintOnEnable: script was enabled");
    }

    void Update()
    {
#if UNITY_EDITOR
        Debug.Log("Editor causes this Update");
#endif
    }
}

Note: OnEnable cannot be a co-routine.
Additional resources: OnDisable.