Endemism, or an Endemic Species is "a species which is found only in a given region or location and nowhere else in the world". To further that definition you must define the size of the region where a species is endemic. It can be endemic to a small scale location, such as a particular mountain top or forest tract, or on a large scale, such as endemic to an entire continent.
All species are important, but preserving unique species, endemic to small areas, is especially important to preserving biodiversity and genetic diversity. Small scale endemic species are especially at risk to be harmed from the impact of invasive species in their ecosystem because they are typically very reliant on it functioning in a specific way. When an invasive species comes in and changes the ecosystem, even a minute amount, it can spell disaster for endemic species.