The chaos monkey deliberately switches off servers in live environments at random. It takes the pain of disappearing servers and brings that pain forward. By deliberately sabotaging their own systems it created strong alignment for the team to design-in redundancy and automation for the necessary resiliency and reliability in the face of random failures. Training for this randomness helps make stronger, more resilient and fault-tolerant systems and software and keeps your movie streaming so you can keep chilling without interruptions.
The chaos monkey is a great metaphor and trigger to actively work on what life could throw at your system before it happens.