Just a simple text, when user point a mouse on it - the text will appear on a same page with a small popup. How to do this ??

Mediabox does not create tooltips, which is essentially what you are describing. Joomla used to have a tooltip script, but I don't know if this is still available in Joomla 4 or Joomla 5.

Which template are you using? The template might have the code necessary to produce a tooltip.

Yes Joomla 3.10 does. As an example, you can add the following php code in your template's index.php file:

JHtml::_('behavior.tooltip');

Then to create a tooltip, add code like this on the target:

<span class="hasTip" title="This is a tooltip">target text</span>

If you are creating the tooltip as part of a link, then you can add the class and title while you are creating the link, in the Advnaced tab of the Link dialog.

Although the "hasTip" class will not be included in the Classes list, you can type it in and press Enter.

Alternatively, if your are creating the tooltip on a text selection, you can use the Attributes dialog (toolbar button has a "cog" icon) to add the Title and Class to the text selection, which will create a <span> tag as in my example above.