When searching within PubMed, you'll get a page of results, like the image seen below. On the left side of the page, there are filters which you can use to narrow down your results to only the most relevant ones to your needs. The most common filter you may be familiar with is limiting your results by date, but there are many other types of filters you can use.
The filters column in PubMed is long and narrow, but it does not list all the filters that are at your disposal. At bottom of the list you'll see a button that says "Additional Filters."
If you click on "Additional Filters" you will be able to select which filters you would like to have shown on the left side of your results. This just means they'll be the options shown if you want to use them as you search. You can select or deselect them as needed.
