Budgeting period is the time frame for planning your expenses. By default it’s 1 month starting with its 1st day.
Within this period you spent money and after it ends your expenses are clear. Note that previous period’s transactions are never deleted!
It work well with the budgets or expense limits for this month.
If 1 month budgeting period is inconvenient for you or you want to start it not from the 1st day of a month you can change it in Settings. Just look for Budgeting period and Start.