What is Ansible?

Ansible is a radically simple IT automation engine that automates cloud provisioning, configuration management, application deployment, intra-service orchestration, and many other IT needs.

Being designed for multi-tier deployments since day one, Ansible models your IT infrastructure by describing how all of your systems inter-relate, rather than just managing one system at a time.

It uses no agents and no additional custom security infrastructure, so it is easy to deploy - and most importantly, it uses a very simple language (YAML, in the form of Ansible Playbooks) that allow you to describe your automation jobs in a way that approaches plain English.

https://docs.ansible.com/ansible/2.3/dev_guide/overview_architecture.html

or as a short answer:

a simple automation tool without any agent to install on targets to run a succession of configuration tasks using a simple human readable language