Active matter describes materials that are out of equilibrium, leading to interesting macroscopic properties. The most obvious example of this would be a self actuated material, in which the components are able to generate stress at the microscopic level. This can lead to some very inspiring emergent phenomena.
Being fundamentally out of equilibrium, these materials not only provide an interesting playground for theoretical physics, but also describe a large set of materials observed in the life sciences. These examples exist at many length scales, from many molecular motors dividing genetic material during mitosis to a dense group of insects all moving in unison forming a single undulating mass.
In my group we use the tools of theoretical physics and computer science to help understand how nature utilizes active materials to achieve specific goals.