Stemming extends a search to cover grammatical variations on a word. For example, a search for fish would also find fishing. A search for applied would also find applying, applies, and apply.
Fuzzy searching will find a word even if it is misspelled. For example, a fuzzy search for apple will find appple.
Select a fuzziness level from the drop-down menu to set how many characters off a word can be from your search term and still count as a hit.