A sinking fund is a small savings pot for a specific expense you can see coming. The name comes from old corporate finance, where companies set aside money to retire a bond, but the household version is simpler: you know the car insurance is $1,200 in June, so you put $100 aside each month starting in July. When June comes, the money is there and nothing in the budget has to give.
It is different from an emergency fund. Emergencies are unknown; sinking funds are for the known. Annual subscriptions, property tax if it is not in escrow, holiday gifts, a wedding, a new laptop, a vacation, a pet's yearly vet visit, the deposit on a lease. Any bill that arrives once or twice a year and would otherwise land on a credit card is a candidate.
The effect is to turn lumpy expenses into flat ones. Someone with $6,000 a year of irregular bills who saves $500 a month never has a bad month because of them. Without the sinking fund, those bills show up as a $1,500 surprise in March and a $2,000 surprise in December, and the surprise is what breaks budgets.
Most people run several sinking funds inside one savings account, tracked on a spreadsheet or in an app, or use a bank that offers labeled sub-accounts. In Allocate, a sinking fund is simply a goal with a landing date, and the monthly amount is worked out from the date.
The common mistake is raiding it for something else and then being surprised again when the original bill arrives.