Human brains have a limited amount of processing power. We can run out of memory like computers. When the amount of information we get exceeds our ability to handle it, our memory suffers. We forget things and constantly put a lot of mental effort into concentrating. It causes cognitive load.
When talking about UX design, the cognitive load caused by a user interface means the number of mental resources required to operate the system.
Intrinsic cognitive load is the effort of absorbing new information. Such a kind of cognitive overload is caused by the complexity of a product and its features. Good user interface design can significantly improve the situation. However, the more complicated a product or mobile app is, the greater the intrinsic overload it may cause.
Extraneous cognitive load means processing that takes up mental resources but doesn’t actually help users understand the content. That’s where designers can improve the situation. Strive not to overload a user with superfluous decorative UI: distracting animations, misunderstood interactions, and the excessive amount of pictures — all this can negatively affect usability and cause cognitive overload.