Modernism is a concept, feeling, period, etc. that is mainly associated with the late 1800s and the early 1900s. It can be characterized by a throwing off of the previous period's more objective reasoning and turning more toward a subjective analysis. Modernism also puts more focus on the struggles of man internally. Much of the literature of the time doesn't necessarily focus on outward conflict, but internal conflicts of the main characters. Authors such as Faulkner and Hemingway appear during modernism as well as artists like Picasso.
Another key characteristic of modernism is stream of consciousness and narrative fragmentation. These could be related to Freud's emerging work in the field of psychoanalysis. Freud himself thought that the human psyche was fragmented into three parts: the id, the ego, and the superego. With Freud's popularity and emerging work influencing the culture of the time, it fits that authors would also use narrative fragmentation when writing and that artists would try to paint from various "angles" (Picasso).