Q: How can I see text in written in description in the slide show?

...earlier I could see that text when I look at the picture in my slideshow!

Have you been using iPhoto or Aperture earlier?  In Photos the descriptions cannot be added automatically to a slideshow.  You have to add text slides manually.

Select a slide and click the "+" button at the right end of the filmstrip in the slideshow. Then select "Add text" and type the description you want to add.

The instant slideshow from an album will show the name of the album as the title, so you would need to rename the album.

Or, if you start the slideshow from the Places view or a moment, clicking the play button ▶︎ to the right of the name of the moment will add a text slide with the location:

I don't really understand what you try to explain...

It is not documented very well how the the title of an instant slideshow of selected photos will be picked.

Which version of Photos are you using?  My screenshots are from Photos 2.0 on Sierra.

• When I select an album and play it as a slideshow the name of the album will be used as the slideshow.
• When I open a map from the Places view and select a place on the map to play the slideshow, the name of the place will be played as the title slide.

But Photos ha no way to use the titles added to the photos as a title in the slideshow like we could do in iPhoto or Aperture.

. But what I want is, when a picture have a text in the description row, that text on the selected picture inside the slideshow should show up on that picture in the slide show!

As I tried to explain, no version of Photos can do that at all, unless you create a slideshow project and manually add title slides and copy and paste the titles into the text slide.  This was much easier in iPhoto or Aperture.

You're welcome

You may want to send a feature request to Apple, if you have not yet done so: Feedback - Photos - Apple