The International Space Station always has a commander that takes a leader role for the crew on board. They ensure that the crew is safe, doing well, and work together as one team.
The commandership is handed over in a ceremony, where the former commander gives a little speech, hands over the key to the Space Station and the new commander talks about their new responsibilities.
Several European astronauts have been commander of the Space Station, from the first one in 2009, Frank De Winne, through Alexander Gerst, Luca Parmitano, Thomas Pesquet, Samantha Cristoforetti and Andreas Mogensen.