then you are not doing it properly.
if you notice in the information about the description field it says that if you are working with a category the description field is not used.
the description is only for forums, not categories.
you have a category, inside that category you create forums with that category as the parent.
change the name of the default category,
now , click on the category name and you will see the default forum.
click the edit button( the green cog ) and change the name of the default forum,
put in a description. submit.
this should help:
https://www.phpbb.com/support/docs/en/3 ... rt/forums/