Basic Tooltip Example

using UnityEngine;
using UnityEngine.EventSystems;

public class Tooltip : MonoBehaviour, IPointerEnterHandler, IPointerExitHandler {
    PointerEventData data;

    public RectTransform tooltip;

    void Start() {
        tooltip.gameObject.SetActive(false);
    }

    void Update() {
        if(data != null)
            tooltip.position = data.position + new Vector2(0, -tooltip.sizeDelta.y);
    }

    public void OnPointerEnter(PointerEventData eventData) {
        data = eventData;
        tooltip.gameObject.SetActive(true);
    }

    public void OnPointerExit(PointerEventData eventData) {
        tooltip.gameObject.SetActive(false);
        data = null;
    }
}