Perhaps an obvious statement. However, it’s important to realize that there are two different ways this manifests and can cause us to make bad decisions when creating or redesigning a site.
Cognitive load is the total amount of mental effort being used out of the capacity of short term memory. Without going too deep: the more things on a visitor’s mind, the less they retain, and the shorter their attention span.
One of the primary reasons visitors leave your website is what we call “visitor fatigue.” The term is often applied to museum visitors, and is a constant focus among curators.