Chillout is a very general word, and for good reason. Ranging from loungecore to chillhop, ambient music to smooth jazz, anything intent on relaxation of the listener can be referred to as `chillout' music. This effect is often achieved with slow tempos and rich timbres, with soft drum hits in the place of hard, transient sounds.
It is difficult to point in the direction of specific artists to listen to as one always seems to fall short of encompassing the diversity within the genre, but some good starting points would be artists such as Owsey, 4lienetic, Izzard, MARO, ford., Direct and Floating Points.