Meaning of "living within means"
In this context, means refers to a person's assets and/or income. So, to "live within your means" refers to not spending out of proportion with your income.
Ask a literary-inspired question, get a literary answer:
Annual income twenty pounds, annual expenditure nineteen nineteen six, result happiness.
Annual income twenty pounds, annual expenditure twenty pound ought and six, result misery.
(Mr Micawber, in David Copperfield, by Charles Dickens)