In Ars Magica 5th, the Character Sheet has a space for a Description with each of the Characteristics. But I can't find in the book what is it for.
These are for specialities.
From Ars, 4th Edition:
A speciality is simply a word that describes each non-zero Characteristic your character has, giving more definition than a statistic like "+2" or "-1" can. For example, if you have a +1 Strength, is your character "thick," "bulky," or "forceful"? Does your -3 Dexterity indicate that he is "bumbling" or simply "clumsy". Characteristic specialities help you visualize your character, and other than that, have no mechanical function in the game.
However. Examining the 5th edition book in detail I can't find any reference to "Description" at all (Checked all index'd references and other places I can think of, not read the whole thing again mind you!) so it looks like this item from 4th edition was lost in the editing from 4th -> 5th as far as I can tell. So, if this is the same as 4th it's purely for flavour and I can't find anything to contradict this (as I can't find anything that actually describes it) so it looks like the 5th has missed this, indeed in the character conversion section (p.224 ars 5th) it says
Characteristics: Characteristics are unchanged between Fourth and Fifth Editions
So I'm pretty confident that it's remained the same.
My guess for this appearing in the corebook is that they anticipated reintroducing them in some form so left that spot available.
What isn't a guess is that they do have a function in the expanded rules...for characters that choose to make use of them.
There are virtues and flaws in some of the expansion books that grant a keyword with a mechanical change to a Characteristic. Specifically they appear in Realms of Power: Magic...but I'm sure I've seen them elsewhere.