We've all heard that men can't multi-task, but women can (you just have to look at the image above to think that's true!), however, the ability to multi-task is actually overstated and that's what Miller's Law is all about.
In 1956, George Miller, a professor at Harvard University, asserted that a person's immediate memory is limited to around 7 pieces of information (plus or minus two), which means our brains just can't cope with too much going on at any one time and the more it does have going on, the greater the likelihood it will fail in the tasks.
Miller’s Law is the reason why when we design larger tasks in our products we decide to chunk them up into smaller pieces with fewer parts. Fewer parts means we come in under the seven pieces of information that most of us can cope with.
This is particularly key when you are addressing the needs of first-time users, as these users haven’t had the time to encode the information presented to them into long-term memory, and thus are fully processing everything we throw at them and not relying on learned memory.