The meaning is:
To register your courses, select the sections that you desire to choose at that particular moment in the Find column and click Submit.
The noun sections is "modified" by desired, and this modification makes the noun definite in this particular context.
You can say "your desired sections":
To register your courses, select your desired sections in the Find column and click Submit.
The word your will occupy the slot that the occupied. Both your and the serve as "determiners" to the noun "sections". The word your will basically mean the same as the in this particular context. Since we cannot combine two "main determiners" like the and your (we can't say "the your desired sections"), we only use either the or your.
Compare with a sentence I've composed:
To make a flower bouquet for my girl, I pick red flowers in the field.
By not using any determiner before "red flowers", I signal to the reader that I pick any red flowers I might find attractive enough, not some red flowers I selected beforehand.
In on-screen messages, articles may be omitted in order to economize screen space:
To register your courses, select desired sections in the Find column and click Submit.