Runners are machines (typically containers) where your jobs run. Gitlab offers shared runners, and you can also bring in your own runners (machines that you register against Gitlab to run your jobs).
The main difference between Gitlab shared runners and the ones you bring is that the shared runners are, well, shared, so sometimes you have to wait for a bit for a job to start. You can also bring more powerful machines so that your jobs run faster.
You can find some docs on runners here.
In your case it seems that the runners have a problem, looking at the exclamation sign next to the name, which should be a green ball. Maybe hovering over the sign will give you some clues into what's happening.