Cognitive Load is a psychology concept closely related to Miller’s Law. It includes the amount of mental resources needed to understand and interact with an interface.
Miller’s Law explains that the average person can only keep memory of 7 times. By overcrowding your design with content and not breaking it up into sections this can overwhelm the user and make them not want to use your product.
As designers we mustn't use the magical number seven to justify unnecessary design limitations, but we must ensure that our content in broken up into chunks to help users process, understand and memorise easily. Using headings and subtitles will also help break content up into chunks to allow the user to remember easily. It is also important to remember that short term memory capacity will vary based on the individual, so not everyone will be able to hold as much information the same as others.