'Compensation' is when they allow you to gain the credits for a failed unit by virtue of your passing x amount of credits first time. It's them saying 'well because you passed this other stuff, we will let you get away with failing that one'.
Each university has a slightly different take on it (sometimes it differs by department). From what they are saying, BY150 sounds like a prerequisite to BY263, which means that in order to study BY263, you must have taken and passed 150 in the prior year.
What this further means is that if you pass by compensation BY150, you have not technically passed it - it has been awarded to you despite a fail. In turn, they will not then allow you to fail BY263, as your compensated pass for this topic has been used already.
It's to make sure you have the necessary skill and knowledge in a topic area and don't used skip through 3 years fo a topic area having failed all of the examinations in that topic area (in this case, Genetics and Molecular Biology).