01) Access the Control Panel
02) After login click on Tools >> Text Administration
03) After clicking on New File
In the Text type field select Text
Select the checkbox Add in the description of all the products
Click on categories and sub-categories with the CTRL key pressed to select more than one option
In the Title field of the Text fill in a title
In the Text Description field, describe the text that you want
After clicking on the INSERT button
Automatically all products of the selected categories or sub-categories will be with this title and text in the description.