Monoliths are a truly alien life form, existing in a higher realm dubbed Spacetime-6. In our dimensions, Monoliths generally take on the form of massive stone blocks or wooden trunks, carved or natural.
To human eyes, Monoliths generally take on the form of massive stone blocks or wooden trunks, carved or natural. They can weigh anywhere from a few hundred pounds to a hundred tons. They can be found all around the Earth. On the Canadian tundra, they take the form of large totems, on the Salisbury Plain in England they have the appearance of enormous upright stone slabs, in the Mojave desert in California they are rocky pinnacles, and on the cliffsides of Rapa Nui they were carved into gigantic stone heads. Most planets in the Federation and probably beyond have similar natural/artificial formations scattered across their surfaces. For example, the giant tufa on Vulcan's Forge and the frozen helix spires on Andor.
Monoliths exist in three dimensions of space and three of time in a higher realm dubbed Spacetime-6. They have no power of independent movement in the space dimensions, but can move freely in all three dimensions of time. Because of this fundamental difference, humans, with only a very few exceptions throughout the ages, are unaware of the Monolith's true nature. Ancient peoples, recognizing spiritual power in them, have used them for millennia as elements in temples or as totems, carting them at great cost all across the countryside. Some shamans and various other rare humans, sometimes through the use of hallucinogens and secret rituals, have learned to traverse the time dimensions and converse with the Monoliths.
Monoliths make no attempt to keep their existence a secret. They are all around us, just almost impossible to encounter. Monoliths communicate through visions and dreams. Through gentle guidance and cryptic hints usually given in the form of visions and dreams, they can potentially affect the course of all history, though the extent of their involvement remains completely unknown. Their area of operation is the entire universe and beyond, apparently.