This is a fee that some accounts may charge if the balance or average falls below a certain threshold.
Steps
Not every account has a fee, and not every account has the same amount.
Ask: What is the minimum threshold, how is it calculated, and what is the fee if it falls below it?
Sometimes an account without a minimum threshold is more economical.
The figures, fees and limits in this answer vary by bank and change over time — confirm them with your bank or the official source before acting.