It is a type of small satellite. For its use, you will need some other things like a computer, a battery, some solar cells, a radio and an antenna also.
Prior to 2013, most CubeSats were built by schools. Today, most originate with commercial or amateur entities.
Even a searchable database of small satellites is available.
You’ll see that CubeSat uses the U Designer to indicate its size. Each U is a cube that is 100 mm – about 4 inches – on one side. So a 2U satellite will be the same size as two 1U satellites next to each other.