Alfresco means outdoors. You can call your casual backyard picnic an alfresco dinner party to make it sound fancier.
An alfresco wedding reception is held outside, as is an alfresco graduation ceremony. Most people use the adjective alfresco to describe a meal, like a picnic or barbecue, that you eat out of doors in fine weather. The word can also be spelled al fresco, which is the way it's written in its native Italian, in which it means "in the fresh air."