Attention, memory and perception
Recognition over recall
It's far easier to recognise something you see than to remember it unprompted, so interfaces should show options instead of making people remember them.
Picking from a list needs much less effort than typing from memory. A visible menu, a recent-files list or a suggestion as you type all let people recognise rather than recall.
Keep options, examples and earlier choices in view, and don't make people carry information from one screen to the next.
In practice
A search box that suggests recent searches and popular items as soon as it's clicked, instead of waiting for people to remember what to type.
Common mistake
Hiding everything behind icons with no labels. An icon only helps recognition if people already know what it means.
Where it comes from
One of Jakob Nielsen's ten usability heuristics, published in 1994, built on long-standing memory research showing recognition is easier than recall.