What exactly is a runner?

What exactly is a runner?

I followed this instructions and I was able to install a runner in my gitlab. so practically what I have is this :
https://gitlab-runner-downloads.s3.amazonaws.com/latest/deb/gitlab-runner_amd64.deb

Then I took a closer look and I found out that this runner seem to be a a mini linux inside my linux just that curl for example is compiled to not support sftp :))

beside the fact that I must download it from bezos why is sftp disabled in the gitlab runner?

GitLab Runner is little bit more complex to fully explain here. Please see the docs GitLab Runner | GitLab