Font stacks are used in CSS to tell the browser which fonts to use, and in which order.
font-family: "Segoe UI", Segoe, Tahoma, Geneva, sans-serif;
For example, in the above CSS tells the browser to try to display the text first in Segoe UI, then if the computer doesn’t have it, try Segoe, Tahoma, and Geneva, in that order. If the computer has none of these fonts, it will use the default sans-serif font defined in the browser’s preferences.
It’s important to consider the fonts in that list. Revised Font Stack suggests some better font stacks. The goal is to include fonts that are similar in weight, x-height, and other type qualities. The article is a good read for both designers and developers looking to understand the options available and how they play together.