Calais is right at the entrance of the Chunnel (tunnel under the Channel between UK & France) and a port for ferries to England.
Migrants, willing to get to UK illegally are driven there by smugglers who will try to make them cross the border. The crossing is dangerous as both police any migration services are controlling very strictly. One needs to attempt the crossing a numerous time and may be, once, be lucky. Or not. Migrants often die trying to cross the border, most of the time anonymously.
Wainting for the attempt, migrants are staying in a "bush" by the city in inhuman living conditions. The hope for a better life and the accumulated debt of their family will make them try until they succeed … or die.