What exactly is a singleton

what is it?

Internet is full of knowledge, suffice to use google Singleton.

Here is an implementation example for Unity:

public class SoundtrackController : MonoBehaviour
{
    static SoundtrackController singleton;
    void Awake ()
    {
        if( singleton==null )
        {
            singleton = this;
            Debug.Log($"{nameof(SoundtrackController)} created");
        }
        else
        {
            Debug.LogError($"The can be only one {nameof(SoundtrackController)}!",gameObject);
            Destroy( this );
            return;
        }
    }
}