It would help if we knew the structure of the tables (or the key they share).
Anyway, your syntax will create a cartesian product, as you haven't set any criteria for the join you are performing (you are performing a join, but with the old syntax and without the on.. part)
So, you would right it as:
select distinct course.course_no, description
from course inner /*(or left or right or cross or full) */ join section
on course.field = section.field --you need to change the field appropriately
where location = 'L210'
order by course_no