GitLab Runner - What is it and why do I need it?

The following article: https://docs.gitlab.com/runner/

Being new to GitLab CI/CD, it is not clear what GitLab Runner is, or why it is needed.

• What is GitLab Runner? What does it do and what problem does it solve?
• Why do I need it? Is it a requirement to use it? Are there advantages to using it?
• Can multiple instances of GitLab runner be setup?

It would be nice if there was an overview describing these things for someone completely new.