Krasko was a mass-murderer from the future, claiming to have killed around 2000 people. When he was released from the space prison, Stormcage, he used a vortex manipulator to travel to Earth, specifically the year 1955, to change the past.
He arrived in Alabama with a weapon that could displace things in time, including people. He couldn’t hurt anyone directly, however, due to a neural restrictor in his brain, so to change history, he aimed to alter small events. He specifically targeted Rosa Parks, whose brave actions kick-started the civil rights movement on Earth.
Because his equipment used Artron molecules, the Doctor and her friends were able to track Krasko and foil his plan. Eventually, Ryan used Krasko’s own weapon against him, to displacing him in time and sending him far into the past.