No, he means a window without the border elements like title bars and such. League of Legends (for example) introduced it a while a go.
It's a nice feature for people that want to use windowed mode without losing precious screen real estate. A borderless window has some nice perks vs. full screen, because of the way Windows handles "full screen" stuff. Switching between programs is fast and seamless in windowed mode, vs. full screen for example.