"Non-GMO" generally means a product doesn't contain genetically modified ingredients. In the U.S. it's most commonly seen as a third-party certification — the Non-GMO Project Verified label is the largest program — rather than a federal standard.
For most fresh produce at a farmers market, the non-GMO claim is redundant: the vast majority of fruit and vegetable varieties sold at small-farm scale aren't GMO anyway (current GMO crops are concentrated in corn, soy, canola, sugar beets, cotton, and a few others). The label is more meaningful for packaged goods and certain commodity categories.