Stemming is a feature to show you more search results that might be relevant to you. For example, if you search for print, it also finds documents containing prints, printer, printed, printers, or printing.
Of course, you might just want pages containing the exact word Print, in which case just un-check the Stemming checkbox!
Some supermarket websites contain this feature. When you search for apples they often show you pages for apple juice, apple cider, apple cake, apple pies before you get to the page where you can buy a bag of apples 🤦
Technically Stemming improves search recall, meaning it will return more search results that might be relevant to you.
Another way of improving search recall is the use of synonyms.