Bioturbation is disruption of soil, sediment, or sedimentary layers due to biological activity.
In the case of animal bioturbation, this is most often caused by recent disruption of the soil or seafloor by small burrowing creatures. It can also be caused by large burrowing animals (see giant ground sloth) and large animals walking (see dinoturbation). In many cases this can be one of the major drivers of surface-layer appearance, and earthworms and parrotfish can poop out whole landscapes.
Most bioturbation, however, is done by plants and fungi, with roots and mycelia penetrating throughout the top layers of the soil. In modern terrestrial ecologies, we simply assume that bioturbation is a given, with a soil biomantle (consisting of the O horizon and A horizon) being present in most ecosystems, excepting the driest deserts and the rockiest ground. We don't usually refer to bioturbation, we refer to soil; in most places, it's the same thing. Because of this, you will rarely see bioturbation referred to outside the fields of marine biology and paleontology.