DPI means "Display Pixels per Inch"... or some refer to it as PPI "Pixels per Inch". What this means is that the value you set determines how many pixels with equal the length of 1 inch. A higher number means you can produce higher resolution images, but since you're doing it for the web graphics, specifically a 120x120 px avatar, 72dpi is web standard; more than enough pixels to get what you want.