Jacob Riis was a Danish-American social reformer and photographer who is best known for his work in documenting the squalid living conditions of New York City's slums in the late 19th century. His concerns were focused on improving the lives of the poor and disadvantaged, and he was particularly concerned with the living conditions of the slum dwellers. He was an advocate for better housing, sanitation, and education, and his work helped to bring about reforms in these areas. The term 'slums' is used to describe overcrowded and unsanitary living conditions, and this was one of the main concerns of Jacob Riis.