One of my favorite critics in the area wrote last Sunday about the term "Modernism" being "...the most problematic for me, and I generally avoid using it since I have never been completely sure I know what it means."
That was surprising. Modernism is an often-used (and abused) term in the art world. If one is looking for a monolithic, clear style or philosophy there, it will lead to disappointment. Anything encompassing from the around 1860 to the 1970's is going to cover a lot of ground, styles and movements. In this case, one of the most prolific periods in Art History. Modernism emerged as a reaction to the obsession in painting with Realism, with the Enlightenment, and Religion.
It was a hopeful time that turned to despair and disassociation after WWs I and II. At times humanistic, peaceful, and with the Futurists (Marinettti), poetic, Fascist, Corporatist and Pro-War. It's complicated, but well-worth learning because it's an historically significant concept, and once understood, quite useful.
For a great first read on Modernism, I recommend "The Shock of the New", by Robert Hughes (it was also a TV series, btw). Click here.