Important stones is not a real concept in Go, but it is useful to distinguish between important stones and their opposite: junk stones.
A useful concept is the subset of important stones called pivotal stones (or key stones). These stones are important because they play a strategic cutting role.
Other important stones are big groups one cannot afford to lose (see also heavy), or stones having influence in a large area. For example, the first stone played in an open corner is an important stone. Depending on later development of the position, such stone can become a junk stone.