A geoid is the irregular-shaped “ball” that scientists use to more accurately calculate depths of earthquakes, or any other deep object beneath the earth’s surface. Currently, we use the “WGS84” version (World Geodetic System of 1984).
If Earth were a perfect sphere, calculations of depth and distances would be easy because we know the equations for those calculations on a sphere. However, the Earth more closely approximates an ellipsoid, which is what a ball looks like if you sit on it. Ellipsoid calculations aren’t as easy as spherical calculations, but they’re still well-known and do-able. Be that as it may, we all know that the earth is not really an ellipsoid because there are oceans, and mountains, and valleys, and many other features that are not part of an ellipsoid.
The geoid is an imaginary sea level surface that undulates (has a wavy surface) over all of the earth; it isn’t just for the oceanic areas, it also extends through the land masses.
You can generalize the relationship between the ellipsoid, the geoid, and the actual shape of the earth with this:
geoid + ellipsoid = Earth
Learn More: NOAA - What is the Geoid?