Martians were an ancient alien species whose society expanded the Milky Way galaxy.
At one point these aliens discovered the secrets of interstellar travel and managed to establish a massive and prosperous colonial empire. They once dominated currently human-inhabited space for millennia before the dissolution of their highly advanced civilization.
The improper nickname, Martian, was the result of mankind encountering signs of their presence for the first time on Mars. Aside from Sol, there were many worlds and systems which were in their sphere of governance and in which they left multiple artifacts. The discovery of their astrogation charts of habitable worlds in the galaxy as well as new metals and technologies led to the expansion of the human race across multiple worlds in the galaxy, and the creation of cortical stacks. It is currently unknown what led to the dissolution of this advanced civilization, or what ultimately became of them.
The Martian legacy is one of colonization and technological advancement of humanity- as humans used the remnants of Martian technology to advance through the galaxy.
The remnants of the metals and astrogation charts left by this advanced alien civilization launched the expansion of the human race throughout the galaxy through Needlecasting and helped evolve the human species through the development of the Cortical Stack.
In the TV series, this alien society is referred to as Elder civilization.