Balance, from latin „bi-“ and „-lanx“ meaning two scale pans.
Generally, to balance something means to bring s.th. into harmony, steadiness, or stability.
There are many things in different areas of your life and body that need to be balanced. Think of e.g. the basic psychological needs for autonomy and attachment, or Na/K in your cells.
And just as equilibrioception (your sense of balance) prevents your body from falling over, striking a balance between stress and relaxation in your grad school life can play an important role in maintaining your physical and mental well-being.
There is no need to be well-balanced at all and every time, but try to keep the balance in the longer term. A PhD certainly has lots of stressful phases - so it’s good to have phases of rest and recreation!
(Reminder: You don’t have to earn them first.)