Being on a quarterly basis means that something is set to occur every three months. Every year has four quarters, so being on a quarterly basis means a certain event happens four times a year.
The calendar year is divided into four quarters, which are often abbreviated as Q1, Q2, Q3, and Q4. The first quarter starts from January 1 and ends on the March 31. The second quarter is from the beginning of April to the end of June. The period between the beginning of July to the end of September is the third quarter, while the fourth and final quarter of the year lasts from the beginning of October to the end of December.