Complex sentence: By definition, the amount of self-information contained in a probabilistic event depends only on the probability of that event: the smaller its probability, the larger the self-information associated with receiving the information that the event indeed occurred.
Split into simpler sentences: By definition, the amount of self-information contained in a probabilistic event depends only on the probability of that event. More specifically: the smaller this probability is, the larger is the self-information associated with receiving information that the event indeed occurred.