The word can refer to a number of things.

First and foremost, it refers to the theory that natural variation in a species leads to some individuals being more successful than others, and that those whose variation gives them an advantage will be more likely to survive long enough to reproduce. That variation will be passed to their offspring, giving them the same advantage. Over time, these variations can build up to produce entire distinct species. It's one of the most solidly supported scientific theories in history.

Seecondly, it's shorthand for something called "Social Darwinism". This is a pernicious distortion of Darwin's ideas, applying them to human society, politics and economics, claiming that "The strong should dominate the weak!". Although the idea bears Darwin's name, he never proposed it, supported it, or had anything to do with it. The concept first appeared in Europe around 1880, only two years before Darwin's death in 1882, and developed entirely seperately from Darwin's own work. It is generally considered to be based on a total misunderstanding of Darwin's theory.

Thirdly, it's used as a pejoritive by fundamentalist Christians who seek to discredit Darwin's theory. This form of 'Darwinism' is a ridiculous strawman that ignores all the subtlety and elegence of Darwin's theory. For example it is claimed by these people that Darwinism claims that an explosion created order, or that species can only evolve by adding fully-formed organs in a single generation. This is a charicature of Evolution by Natural Selection, which can be safely ignored.