For the uninitiated to D&D lore, the humanoid tentacle dudes are called mind flayers, also known as illithids. They procreate through a process called ceremorphosis, which involves what you see in the cinematic: the insertion of a mature illithid tadpole into the brain of another humanoid being, usually humans. They primarily reside in the Underdark, but are feared across the multiverse: they sail the Astral Sea/Astral Plane with their gross ass ships you see in the cinematic, and are often depicted as being at war with the Gith, the yellow skinned people you see in the cinematic as the prisoners of the mind flayer.
In short, mind flayers are evil psychic aberrations that exist across the multiverse and want nothing more than to wrap their super gross tentacles around your head and deal 10d10 damage to your face as they attempt to suck your brains out of your head. Approach with caution.
Also, this only scratches the surface with mind flayers. There's also elder brains, which are massive brains that sit in gross pools that tadpoles grow in, and alhoons, which are mind flayers outcast from illithid society because they delved into arcane magics to achieve a form of lichdom (undeath)...
Mind flayers are great, folks.