When you read "full sun," it means that a plant needs direct, unfiltered sunlight for at least 6 hours a day. This is the minimum amount of light a sun-loving plant needs to thrive. As you have noticed, the intensity of sunlight varies depending on the time of day. A plant that gets full sun all morning but is shaded in the afternoon has a much different growing environment from one shielded from the sun in the morning but exposed to full sun in the afternoon.
Many plants that are classified as growing best in "partial shade" can take full morning sun, as long as they are protected from direct afternoon sun. Latitude and elevation play a role too. Gardens in the South receive more intense sun than those in the North. And gardens at high elevations are brighter than landscapes at sea level.