LOL: What is a roaming mid laner?

{League of Legends Esports Article}
3 packs of 1, 4 packs of 2, all kinds of roaming support are Lin Ran’s special skills
Moreover, he is extremely good at farming for growth, and can use wild monsters as his own line of soldiers, making his own bastards miserable.
Lin Ran, who debuted in S6, worked hard to break the darkness before dawn, and together with his teammates, held up the light of hope for the LPL.