It stores a period of time using year and month datetime fields.
For example,
1.
INTERVAL '235-2' YEAR(3) TO MONTH
This indicates an interval of 235 years, 2 months
2.
INTERVAL '123' YEAR(3)
This indicates an interval of 123 years, 0 months
3.
INTERVAL '354' MONTH(3)
This indicates an interval of 354 months