Safari Smart Search Field lets you type in either an URL to visit or a search query to send to the default search engine. It's also a place where search suggestions automatically pop up as you type. With this cool feature, you can type just the first few letters of a query and rely on the search engine (Google, Bing, Yahoo, etc.) to help you complete the search term without needing to type out the rest of your query. But, certain users may not be fond of this feature for privacy reasons.
If you're among them, use the step-by-step instructions provided in this tutorial to turn off suggested search terms in Safari for iPhone, iPad, and Mac.