The asthenosphere is located between 623 and 124 miles below Earth's surface. It is under the lithosphere. This layer is responsible for plate tectonics, due to its high temperatures and the fact that it is plastic and allows seismic wasves to pass through. The lithosphere is the crust and the upper most part of the mantle. It is at the uppermost part of the asthenosphere where the plates of the Earth's crust move, due to the heat and pressure in this layer, the rocks above this layer behave elastically and eventually can break, which causes crustal movement, fault lines, etc. There are convection currents that cause heat to flow from the Earth's interior, towards the surface. This energy is responsible for the movement of the crustal plates. You can envision a bowl of soup with croutons floating on top of it to represent successively, the asthenosphere and the lithosphere.
The lithosphere is the uppermost solid part of the mantle. It is not as solid as the outer layer, the crust, but it is not as liquid as the next layer of the mantle, the asthenosphere, which lies directly beneath it. Here, the rocks have a quality called plasticity, where they tend to flow more like liquids. After that, you have the rest of the mantle, which is of a true liquid-like melted rock nature.