This article might have further canon details available on.
Mi-go communicate by clicking their pincers and changing the colour of their heads; this sign language can be learnt but usually not 'spoken' by other creatures. A mi-go can speak in a strange, buzzing voice, but only when forced to do so. Mi-go do not need to breathe and can fly through outer space at an incredible speed. Their claws are capable of swiftly and painfully performing surgical operations upon hapless victims.2
Mi-go invade and colonise planets for various purposes, from strip-mining them to seeking resources for scientific experiments. They are usually viewed as gods or demons by the primitive inhabitants of these planets, a fear that they exploit to the fullest to disguise themselves and infiltrate and harvest these societies. They usually hide their core bases of operations in remote, defensible places.2
Mi-go technology merges magic and technology in strange and unsettling ways. Most mi-go worship Shub-Niggurath, seeing her fecundity as the pinnacle of their own biological technologies and skills. To a mi-go, science and faith are largely the same.2
The Uskheart Caves of Nidal are home to a mi-go colony that has long been allied to the Shades of the Uskwood. In exchange for permission to mine resources rare on their world, the mi-go whisper blasphemous secrets to the druids, the latest of which is the location of a hive queen in the Uskwood.3
For additional as-yet unincorporated sources about this subject, see the Meta page.