Cellularity refers to the percentage of cells growing in her bone marrow. Cellularity starts at 100% at birth and decreases with age, but 10% is low for a 68-year-old. 30% would be more typical at that age. Low cellularity is the one of the factors that can produce a diagnosis of severe or very severe aplastic anemia.
Since your mom has already been diagnosed and is undergoing treatment, what's important is the success of the treatment, how her blood counts respond, and the results of her next bone marrow biopsy.
Feel free to search these forums for other relevant discussions. For example, you'll find a discussion of cellularity in a thread named
Marrow Cellularity %? and user "amykdb" spoke of her mom's low cellularity
here.