What does "against which it must be seen" mean?

A sentence from TOEFL reading: " The best signal depends not only on its brightness but also on how well it contrasts with the background against which it must be seen."

My question is, I can not understand the rear part of the sentence:"it contrasts with the background against which it must be seen." Specifically, what confuse me is "against which" and model verb " must". Please help me out. Thankyou!

Solution 1:

A signal is seen against a background. For instance, an aircraft's navigation lights are seen against the background of the sky. And this background is "the background against which the signal must be seen".