Cash flow
The money moving into and out of your accounts over a period, and the difference between the two. A positive figure means more came in than went out.
Cash flow is a flow, measured across a stretch of time, where net worth is a snapshot taken at an instant. A household's monthly cash flow is take-home pay plus any other income, minus everything that actually left the account that month. The gap at the bottom is what is available to save, invest or put against debt.
It is the number that decides whether a plan is possible at all. A budget that assigns more than the month brings in fails on contact, whatever the annual totals suggest. Lenders ask a narrower version of the same question when they work out a debt to income ratio, which looks only at required debt payments against gross income rather than at the whole picture.
The mistake is averaging. Timing matters as much as totals: an annual insurance premium, a quarterly tax bill and a car service land in particular months and nowhere else, so a household can be comfortably positive across a year and short in March. Spreading the year evenly hides exactly the months that cause trouble, which is the gap a sinking fund is built to close.