Too many choices can be mentally difficult to process. So when confronted with too many choices, humans tend to go with the default option or to defer (put off) making a choice - even not buying a product or doing an action. Too many choices have also been associated with unhappiness and decision fatigue.
Examples:
A person wants to switch kitchen cleaners for an environmentally friendly brand. They walk into the cleaning products aisle and see many eco-friendly choices. It's hard to choose one, so do not buy anything.
A woman wants to start on a family planning method and is provided with information on ten methods at once, so she does not choose one.
What it is not:
It is not a reason to eliminate all choices - options are important for clients!