I would like to show up the description before beginning the survey. I've created one description under text elements but it does not display on the token page, what is wrong
What version of LimeSurvey is used? The token screen has a separate theme page template. The description variable might not be placed in that theme page.
As Jelo said: Read about and afterwards use the template editor.
Usually the description and the welcome message are in the welcome screen.
So you find the following in "welcome.twig"
Just copy these lines to the place you like in the "token.twig"
Joffm
Volunteers are not paid.
Not because they are worthless, but because they are priceless