What's easy to process appeals to us more.
The easier something is to process (a scent, a piece of text, a decision), the more you like it, and that changes what you choose next. Processing fluency is how easy it is for the mind to process what's in front of it. Something can have many options and still feel simple.
A study rotated a store's ambient scent across three conditions throughout the day: a simple scent (orange), a complex scent (basil, green tea, and orange), and no scent at all. The two scents had previously been rated equally pleasant despite differing in complexity, yet customers spent significantly more money when the scent was simple than when it was complex or absent. The only real difference was how easy each one was to process.
When a stimulus (a scent, a piece of text, an interface) is easier to process, we like it more, and that shapes the decisions that follow. The effect comes from how little mental effort it takes to understand it, and it shows up even when the number of options stays the same.
Source: Herrmann, A., Zidansek, M., Sprott, D. E. & Spangenberg, E. R. (2013). The Power of Simplicity: Processing Fluency and the Effects of Olfactory Cues on Retail Sales. Journal of Retailing, 89(1), 30-43.
To simplify a message, a form, or an interface, it's sometimes enough to make it easier to process, without removing actual content or options. Clearer language, an obvious visual hierarchy, or fewer steps to reach the same result can do that.
Related concepts