Openstack Heat is an "orchestration tool" which is used to define a collection of virtual resources (Servers, networks, volumes, firewalls etc) which should be created in bulk. To define such a collection of resources you use text-files in a YAML format called the "Heat Orchestration Template", or HOT files. When a collection of resources is defined you can create one or more instances of that collection. A typical use-case can be to create a HOT-template describing a single lab-instance for a student-group, and then instantiate it multiple times to create multiple identical LAB scenarios.