An open circle at the end of an interval means that the end point is not included in the interval, i.e. strictly less than or strictly greater than. A closed circle means the end point is included (equal to).
Open circles are used for numbers that are less than or greater than (< or >). Closed circles are used for numbers that are less than or equal to and greater than or equal to (≤ or ≥).
Answer: An open circle at the end of an interval means that the end point is not included in the interval, i.e. strictly less than or strictly greater than. A closed circle means the end point is included (equal to).
