Simple search is the main search bar you'll see when you first visit PubMed. It's intended to be used like a google search - simply enter your terms as they are. No need to add AND or OR operators for this kind of simple search.
This can be a helpful place to start. You can see what might be out there and use the results to inform a more advanced search strategy if you need one!
By default, your results will be ordered by "Best Match," or relevancy, and presented in groups of ten. You can configure the display by clicking "Display Options," and selecting a different sort order (date, author, etc.), and the number of results per page.
You can also change the default "Summary" format to "Abstract," if you'd like to skim the abstracts for each reference without having to leave the results page.
Filters are available on the left hand side to further limit your results by date, study design, article type, language, and more. This can be particularly helpful for locating review articles!