The daily budget
The part of the week you allow yourself per day.
The daily budget is a share of your weekly limit: 15 % by default. Spend about that each day and the week lasts.
To change it, open Settings with the gear and set the budget anywhere from 5 to 50 % of the week. The number in the centre and the Today row follow the new value at once.
Codex Free has a 30-day window instead of a week, so the budget is scaled to it: 15 % of a week is 3.5 % of 30 days.