Kinda short answer just the bare necessity:

The Gutenberg discontinuity lies at the boundary between the mantle and the outer core. At around 2,900 km (1,800 mi) below the surface, an abrupt change occurs in the seismic waves (generated by earthquakes or explosions) that travel through Earth. At this depth, primary seismic waves (P waves) decrease in velocity while secondary seismic waves (S waves) disappear completely. S waves shear material, and cannot transmit through liquids, so it is believed that the unit above the discontinuity is solid, while the unit below is in a liquid, or molten, form.

Kinda long answer but makes more sense:

The Gutenberg discontinuity is observed at a depth of about 1,800 miles below the surface. At this depth, there is a sudden change in the velocity of P- and S- seismic waves as they travel. At 1,800 miles, P-waves decrease in velocity while S waves disappear completely S-waves (a.k.a. ‘shear waves’), are not supported by a liquid environment. It is believed that because of the vanishing act of S-waves, that this discontinuity marks the boundary of the boundary between the lower mantle (solid) and the outer core (believed to be molten). The molten section of the outer core is thought to be about 1,292 °F hotter than the overlying mantle. It is also denser, probably due to a greater percentage of iron.

The boundary is often referred to as the core-mantle boundary (CMB) and is a narrow, uneven zone, and contains undulations that may be up to 3-5 miles wide. These undulations are affected by the heat-driven convection activity within the overlying mantle, which may be the driving force of plate tectonics-motion of sections of Earth's brittle exterior. These undulations in the core–mantle boundary are also affected by the underlying eddies and currents within the outer core's iron-rich fluids, which are ultimately responsible for Earth's magnetic field.

The boundary does not remain constant. As the heat of the earth's interior is constantly but slowly dissipated, the molten core within Earth gradually solidifies and shrinks, causing the core–mantle boundary to slowly move deeper and deeper within Earth's core.