In short: why use Ansible, and how does it work?

Automation reduces human error and keeps infrastructure consistent. Ansible does this agentlessly: a control node reads human-readable playbooks and connects over SSH (or WinRM) to put Linux, Windows, UNIX and network devices into the described state.

• Why automate: to reduce human error and quickly make sure the IT infrastructure is in a consistent, correct state.
• What Ansible is: an open-source automation platform that adapts to many workflows and environments.
• What it manages: Linux, Microsoft Windows and UNIX servers, and network devices.
• How you describe the work: playbooks, which are human-readable text files describing the desired state of the infrastructure.
• How it runs: agentless. Ansible is installed only on a control node; it connects over standard protocols such as SSH and runs code on the managed hosts to put them into the specified state.

Go deeper:

• How Ansible Works (Ansible Collaborative / Red Hat) — the official one-page summary of the agentless, playbook-driven model.
• Ansible 101 - Episode 1 - Introduction to Ansible (Jeff Geerling) — first episode of a full free course for going further.