Surtr is a cron job that will terminate the oldest node in your kubernetes cluster. Having really old nodes in your cluster isn't ideal as it can make you vulnerable to malicious attacks and can sometimes hide failure states in your applications. By making sure no node ever lives too long, you both remove the potential security risk and expose any flaws in your cluster/application design. If taking out one node causes a load of errors, you know you need to improve the resilience of your services.
Surtr is intended to be run as cron-job (see the examples directory for the cron and rbac definitions). Surtr needs to be able to query the Kubernetes api for a list of the nodes - it finds the oldest node, checks it's older than the min age specified and then sends an ec2 termination request for that node.