Channels are fluid properties like velocity, age, or pressure that can be used for shading purposes. RealFlow's connectivity plugins and the RealFlow RenderKit are able to read out the channel data and apply them to shaders.
Some export formats offer the possibility of selecting which of the activated channels you want to store with the mesh.
Velocity et. al.
Here you specify which channels you want to calculate with the mesh – the appropriate information is extracted from the fluid particles (please also see → “UVW Mapping” in the “Texture” section). The channel's data can be made visible in the viewport with the mesh node's Display > Particle channel option.