Re: Hmm?
They're talking about a product that's been out for years, so opinions vary on how much they should be explaining what things are when they're not news. When, for example, they described in some detail what ffmpeg was, there was quite some complaint in the comments for assuming that people wouldn't know.
But to answer your question, GitHub Actions is a method for automatically doing things from repositories, like running tests when new code is pushed, making daily builds, making release packages, updating servers that run the code, all the things that are often scripted when things in a repository change. A runner is any machine that executes those tasks under the control of the system, and you can rent those from GitHub or you could connect one of your own and run their software on it. There are quite a few options in this area, generally called CI/CD (continuous integration, continuous delivery/deployment) and several advantages to having them. A basic example is an open source project where code contributions may come through from random people and the CI/CD system can run a bunch of tests on any proposed code; if the code fails one of them, then the submitter is informed and the maintainers whose time is limited don't have to look at it. It's good for the submitter because they get faster results so can fix things without waiting for maintainer review, and it's good for the maintainers because it directs their attention when there might be lots of noise.
A bigger example is a thing that I've worked on which requires various pieces of secret information for cryptography, we don't want to hand those to individual users, but we do want individual users to be able to use them for specifically defined purposes if they pass certain tests. Build that into a CI/CD workflow and the build machine can have the secrets and allow their use only under specific conditions. Before that, the secrets were held by two trusted people, you had to get their manual assistance (minor, but still) to use them, and if both I and my other colleague were not there that day, you couldn't build something with them until one of us returned, a problem they weren't solving because evidently we were the two they were assured enough wouldn't misuse them.
GitHub Actions is just one such system. It's somewhat popular because GitHub is popular and they unsurprisingly have a strong integration with it, but you don't have to use it to get that kind of system. A lot of older software shops have hand-built parts of that, but they're usually less flexible than something the project teams can customize to their particular requirements.