When Mary Beton eats at Fernham, her meal is a non-delicious combination of beef, prunes, and custard. And boy does she have words to say about that.
Here she is on the prune:
If any one complains that prunes, even when mitigated by custard, are an uncharitable vegetable (fruit they are not), stringy as a miser's heart and exuding fluid such as might run in misers' veins who have denied themselves wine and warmth for eighty years and yet not given to the poor, he should reflect that there are people whose charity embraces even the prune. (1.26)
The beef, prunes, and custard are so bad that, sitting with her friend after dinner, she can't even think straight. She keeps returning to the bad meal and what it says about women artists. Two chapters later, she's still thinking about the meal: "Now what food do we feed women artists upon? I asked, remembering, I suppose, that dinner of prunes and custard" (3.12).
So, you've probably figured out that the bad meal at Fernham is a symbol for the bad intellectual food women receive, too. But it's also literally a bad meal—and how can you write with indigestion?
Remember what Mary says in Chapter 1: "One cannot think well, love well, sleep well, if one has not dined well" (1.26). Woolf emphasizes this point to argue that, duh, people's bodies matter. Whether you're cold, hungry, sick, or tired matters. And if you're eating bad food, waking up at night to tend to your family, or denying yourself rest because there's domestic work to be done—you're not going to do good work. You just can't.
We mean, you can barely drive a car under those circumstances—much less produce an eternal work of literature. Right?