If your SEVIS (or I-20) is terminated OR you overstay your grace period, you could be accruing unlawful presence. This can lead to arrest and deportation proceedings. Accruing unlawful presence may result in being barred from the U.S. for many years or even permanently.
Accrual means adding to, accumulating, obtaining, or growing. This means you accumulate or grow days of unlawful presence each and every day beyond your status violation or end of your lawful grace period.
Unlawful presence: The period of time when you are in the U.S. without being admitted or paroled or when you are not in a "period of stay authorized by the secretary."
Barred from entry: Being barred from the U.S. means that you cannot enter the U.S. in any immigration status for a specified length of time. You can receive a 3-year or 10-year bar from the U.S. if you have more than 180 days or one year of unlawful presence, respectively.