Those 'Trim' knobs next to the send and return jacks adjust the signal levels at those jacks. The shrieking noises are probably the result of having one of those knobs set too high.

The series loop sends all of the signal out to the effect unit.

The parrallel loop splits the signal into two (parallel) paths, one going out to the effect unit and back in again, the other path continuing through the amp. This allows you to blend the effected signal with an uneffected signal.