Does endemic just mean 'rare'?

No — endemic describes geographic range, not abundance. An endemic species can be locally abundant within its restricted range; what makes it endemic is that this range, however large or small, is found nowhere else on Earth. Rarity is a separate axis, though endemic species with very small ranges are often more vulnerable to habitat loss simply because there is nowhere else for the population to persist.

Why do islands and isolated regions have so many endemic species?

Geographic isolation — an island, an isolated mountain range, or a continent separated from others for millions of years — prevents species from dispersing in or out, giving local populations time to evolve independently into forms found nowhere else, which is why islands like Madagascar and continents like Australia have unusually high numbers of endemic bird species relative to their land area.

Bird Atlas

Endemic Species

short

An endemic species occurs naturally in one specific geographic area and nowhere else in the world, making it a priority for conservation attention.

Definition

An endemic species is one that occurs naturally in a single, defined geographic area and nowhere else in the world — the area can be as large as a continent or as small as a single island or mountain range. Endemism describes geographic range, not population size or rarity: an endemic species can be locally common within its restricted range, but by definition it exists only there, which makes it especially vulnerable if that specific area is affected by habitat loss, since there is no other population elsewhere to sustain the species.

Endemism is usually expressed at a particular geographic scale — a species can be described as endemic to a country, a region, or a specific island group, and the same species is sometimes endemic at one scale (a single country) while more widespread within a larger area (a whole continent). The regional atlas pages covering countries such as Australia and Kenya highlight species endemic to those areas specifically, distinct from species that simply occur there among a much wider range elsewhere.

Why isolation produces endemic species

High rates of endemism are strongly associated with geographic isolation. Islands, isolated mountain ranges, and long-separated continents limit the movement of species in and out, giving local populations time to evolve independently — sometimes into new species entirely distinct from their nearest relatives elsewhere. Australia's long isolation from other landmasses is a well-known driver of its distinctively high bird endemism, and similar patterns appear on island groups worldwide, from Madagascar to Hawaii, where a large share of native bird species occur nowhere else.

Because an endemic species has, by definition, no other population to fall back on, endemism is one of the strongest predictors used in setting global conservation priorities — a species found in only one small area faces total extinction from a single localized threat in a way a widespread species does not.