Normal lenses are not sealed airtight. When internal elements or groups of elements move, dust or other contaminants can be brought into the lens. The L series Canon lenses offer greater resistance to moisture and dust through the use of improved seals. This reduces the likelihood of moisture and dust getting into the lens. But the seals only reduce, they do not eliminate, the possibility of dust getting inside.
In my experience, the dust is rarely ever a problem, or seen on an image. Large amounts of dust could cause an overall softening of the image and increase the likelihood of flair. A single dust particle is of no cause for concern in almost all cases.
