When something becomes mellow, it gets softer or more relaxed. People are often said to mellow with age, meaning their aggressiveness, their sarcasm, and their short temper have given way to a more easygoing, genial manner.
Wine and other foods mellow just like people, and can be described as "having mellowed" when they reach a full and pleasing flavor, without sharpness. The laid-back quality of jazz pieces like John Coltrane’s “My Favorite Things” is often described as mellow. Sometimes the musicians who play them are mellow too. When people are slightly and pleasantly intoxicated, they often claim to be feeling mellow, having attained a relaxed and agreeable state without even having to grow old.