Native plants are those plant species that evolved in your region without human interaction. In the northeastern US, generally that means plant species that were here before European colonists arrived. Plants from other parts of the world are referred to as “introduced,” “exotic,” or simply “non-native.”
Because they evolved in the local environment, native plants usually need less care (such as fertilizers and pesticides that may harm the environment) than many non-native plants. They usually require less water, too (except for those adapted to growing near water bodies).
No matter how small the space, when you choose a native species of plant instead of a non-native one, that individual action is a step towards supporting healthy local ecosystems and biodiversity. Better yet, when multiple people in a neighborhood or community make those choices, they can combine into thriving "conservation corridors" that support a diversity of vibrant life.
The Native Plants section of Planting Westchester covers most native plants such as perennials, shrubs, grasses, etc. For native trees, see the Trees section; for native plants to grow near streams, ponds, etc., see Planting by Water.