Hotland is a location in Underfell where Alphys' lab is. There are also a lot of monsters there, as well as the Core.
Mettaton lives here as well, under the control of Alphys. He also performs here, but he doesn't have much luck.
It is most likely darker and scarier than the original Undertale Hotland, like the edit shown here.
(Please note that this is not the official UF! Hotland and it is only the artist's interpretation.)