the upper layer of the earth's mantle, below the lithosphere, in which there is relatively low resistance to plastic flow and convection is thought to occur.

The layer of the Earth's mantle described in your question is known as the Asthenosphere. Here are the steps to understand this:

1. The Earth is divided into several layers: the crust, the mantle, the outer core, and the inner core.

2. The mantle, which is the second layer of the Earth, is further divided into two parts: the upper mantle and the lower mantle.

3. The upper mantle is composed of the lithosphere and the asthenosphere. The lithosphere is the rigid, outermost shell of our planet and includes the crust and the uppermost part of the mantle.

4. Below the lithosphere is the asthenosphere, which is the part of the mantle where there is relatively low resistance to plastic flow and convection is thought to occur.

5. The asthenosphere is important because its semi-fluid nature allows for the movement of tectonic plates in the lithosphere above it. This movement can lead to geological phenomena such as earthquakes and volcanic activity.