This error occurs when you have too many records to display in your results. There are several reasons you could get too many records, but the most common is using the "Expand search with related terms" search option when searching scientific or discipline-specific terminology.
We have that option on by default, because it's useful for broad searches using common language terms. It only comes into play when doing keyword searches, so switching to a title, author, or subject search can also improve your search results.
When performing more precise searches, turn off this option from the left panel of search limiters or from the advanced search screen.