If you're working with (monthly) budgets, oftentimes, you're allowed to move unused spend to the following month(s) or deduct overspend from the following month(s), this is called "rollover".
So if this month's (or other cycle's) budget was $1,000 but you spend $900, we will automatically add $100 to the next cycle's budget, but only if you enable rollover, this will be optional per budget.