difficult sentence meaning

You're right. It's a tricky sentence.

Passive nihilists believe in something—anything—because any belief is better than no belief.

Apparently, active nihilists enjoy staring into the abyss.

... because believing in something will be seen by the passive nihilist as preferable to taking the risk of not believing in anything; that is to say, to taking the risk of staring into the abyss – a metaphor for nihilism that appears frequently in Nietzsche’s work.

Here, "staring into the abyss" is just a restatement of "not believing in anything". Structurally, it means the same as this:

... because believing in something will be seen by the passive nihilist as preferable to taking the risk of staring into the abyss, which is a metaphor for nihilism that appears frequently in Nietzsche’s work.