The example above sets the background to be black with 50% opacity. The last value of an rgba value is the alpha value. An alpha value of 1 equals 100%, and 0.5 (.5 for short) equals 50%. We use this method to add transparency to an element without affecting the content inside.
Using the rgba value is most preferable when the background has content like text compared to using the background-color property then going on to use the opacity property. First, it's shorter and second, it eliminates the problem of having the content's transparency change with that of its background, if it's something you do not want.