Quarters refer to the quarter of the year:

Jan, Feb, Mar = March Quarter (1st Q)
Apr, May, Jun = June Quarter (2nd Q)
Jul, Aug, Sep = September Quarter (3rd Q)
Oct, Nov, Dec = December Quarter (4th Q)

It is important to remember this is the quarter in which the event was REGISTERED, which is often, but not always, the quarter in which the event occurred. For example, someone born 31 Dec (born Dec Q) may not have their birth registered until the following quarter (Mar Q).