If a place is uncharted, it means it isn't found on any map, like the uncharted wilderness settlers found when heading west, or an uncharted island inhabited by stranded castaways, or, more figuratively, the uncharted territory of a new strategy.
The word charted originated in the 1570s, when chart was the name navigators used for maps. So when navigators reached a place not found on their charts, they called the place uncharted. Uncharted also can be used in a figurative sense to describe the start of a new experience, such as the uncharted territory of talking about a difficult subject for the first time.
