Native plants are naturalized, or adapted to conditions that are present in a geological location. They are suited to the local climate and soil conditions.
In North America, a plant is considered native if it existed in a location pre-colonization. So for hundreds of years, native plants have flourished without help from people; without sprinkler systems, fertilizers, and pesticides. Texas native plants are adapted to our hot, dry summers, so in your landscape they can thrive in these conditions with little input from you.