Terraform - A Tools to Build your Infrastructure an open-source project Hashicorp labs if you are aware with AWS and heard of CloudFormation both work in same manner but Terraform have some better feature you can write your whole Infrastructure as a Code run it in one click and decommissioned it in one click.
For more you must visit the site: https://www.terraform.io
Now Kubernetes (An open source project by Google) and Apache-Mesos( Or DC/OS) An project by Apache foundation both are used for Container orchestration (and I’m purposely avoiding using the word Docker) is not for everyone and does not answer every need.
Mesos was launched first but it was really hard to manage Mesos networking that time. and In 2014 there was the first Release of Kubernetes comes in.
Now, DC/OS (the Distributed Cloud Operating System) is an open-source, distributed operating system based on the Apache Mesos distributed systems kernel.
It's in the race with Kubernetes .
I would suggest you must go through this article to get a better understanding of Kubernetes vs Mesos : https://logz.io/blog/kubernetes-vs-mesos/
And Yes they are not related to Terraform at all.
Thanks