The image you have shared is of Parisian people observing a moment of silence at the Place de la Republique in Paris, in honor of those who died during the terrorist attacks on the offices of the satirical newspaper Charlie Hebdo. On the seventh of January, 2015, two men armed with assault rifles forced their way into the offices of Charlie Hebdo and opened fire, killing twelve people and injuring eleven more. The men identified themselves as belonging to a branch of Al-Qaeda based in Yemen. Al-Qaedea is an extremist Muslim group known for their attacks on public spaces around the world, which have resulted in hundreds of deaths. The gunmen who attacked the offices of Charlie Hebdo were motivated by the fact that the magazine had published depictions of the Prophet Muhammad, most of them poking fun. In Islam, it is wrong to create images of the Prophet Muhammad, and it is especially shameful to make images which treat him as a joke.
People in the photo are holding up signs with the message, "Je suis Charlie-" which translates to "I am Charlie." This message has become a symbol of solidarity with the people of France who have been impacted by tragedies like the Charlie Hebdo attacks. The photo itself shows numerous people sharing in the sentiment of respect for those who lost their lives and were injured during the shooting.
Images like this one represent a very real social problem felt in Europe and North America today. As radical Islamic groups like Daesch (also called ISIS or ISIL) gain power and territory through use of force, they uproot millions of people from their homes. Many people worry that the refugees of such tragedies who seek asylum in other nations might be bringing radical ideas with them, too. These fears are largely based in cultural misunderstanding and racism, but they've been bolstered by the fact that some foreign terror attacks are carried out by people from the same nation as those seeking refuge. For example, the extremist group Daesch is primarily located in Syria and have uprooted many people from their homes. Some people worry that if Syrian refugees are allowed into their country, they will bring the terrorism with them directly or as a consequence of their being given asylum.