A sub-brown dwarf is something between a planet and a brown dwarf. While brown dwarfs are unable to fuse hydrogen, they still get their energy from fusing deuterium. Sub-brown dwarfs are unable to fuse even deuterium and the only source of heat they have comes from their formation. Their mass can be as high as 13 Jupiters (which is the minimum needed to fuse deuterium) and as low as one Jupiter.
There is not a clear definition that separates sub-brown dwarfs from rogue planets. Scientists argue that planets form accretion disks around stars, while sub-brown dwarfs form from collapsing interstellar clouds. However, there is no way to separate them, as there are many planets with a mass similar to a sub-brown dwarf. A free-floating object, which could be considered a sub-brown dwarf, could be actually ejected from a planetary system, while an orbiting planet could be a sub-brown dwarf captured into orbit. There is no way to tell.
When they are young and hot, they appear glowing red and look similar to brown dwarfs. However, without an internal source of heat and with a low mass to hold heat inside for long, they cool down fast. They might still glow faint in infrared (but with no visual light) for billions of years, as they cool below water's freezing point. Unlike stars and large brown dwarfs, they do not compress into white dwarfs, as their mass is too small for that.
While they are hot and glowing red, they might have a radius larger then Jupiter. As they cool down, they become very similar to Jupiter regarding their size.