If a place is uncharted, it means it isn't found on any map, like the uncharted wilderness settlers found when heading west, 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.