select
method on ActiveReocrd classes override sql SELECT statement used to fetch records from db. The example you gave will execute the query:
SELECT mon_hrs + tues_hrs as two_day_total FROM billable_weeks LIMIT 1
It will return just one column (not really existing in the table), called two_day_total
being a sum of vlueas stored in two real columns mon_hrs
and tues_hrs
.