A Space Elevator is a system for transporting cargo from a planet surface to orbit without the need for large rockets. It consists of a cable (tether) that extends out from the planet into space. It is anchored to the surface of the planet, and the other end is anchored to a counterweight (possibly a space station or asteroid) positioned past geostationary orbit. The cable would allow for a vehicle, called a climber, to travel from the ground to space and back again, delivering cargo or people into space. The force of gravity vs the force of outward / upward centrifugal force would result in the cable being held up under tension, and stationary over a single position over the planet. Depending on the gravitational pull of the planet the Space Elevator is anchored to, the cable would have to be made of a material that has a very high strength to weight ratio.
Part of the Megastructures book project, to be released late 2021 / early 2022: http://www.neilblevins.com/books/megastructures/megastructures.htm