No, not a hallucinogenic drug made from peyote, but rather a potpourri of young, small salad greens. The term apparently comes from the Provençal word for "mixture", and the idea comes from the practice - popular in locales with fields of greens - of gathering young field greens to make a salad. Mesclun has become quite popular throughout the western world, and it can be found in many supermarkets. The mix varies depending on the source, but among those greens commonly included are arugula, dandelion, mizuna, oak leaf lettuce, radicchio and sorrel.
If at all possible, choose mesclun with crisp leaves and no sign of wilting. Refrigerate in a plastic bag for up to 5 days, and don't wash it until you are ready to use it. Because mesclun leaves are tender, they will respond best to being softly bathed in a light vinaigrette.
A nice addition to a simple mesclun salad is a disc of soft goat cheese (a.k.a. chevre), coated with chopped toasted walnuts, pepitas, or even bread crumbs, and heated in a 300°F oven for 5 minutes. Classy.