Reformism is a political doctrine advocating the reform of an existing system or institution instead of its abolition and replacement.
A reform is a small change that results in slightly less harsh conditions for the oppressed. Reforms themselves are good, but when reformism becomes a main political ideology, it holds back real change.
The problem with reforms is that capitalists aren't going to want to submit to social reforms that "come at their expense". They have most of the benefits that the masses want, with an individual "freedom" and power that will not likely exist in a socialist society.
Capitalists can move outside the political system to cripple reformist movements through the private press, political corruption and relocating businesses while claiming that the increasing costs of business make it necessary to do so and thus turn people against reformist causes. This causes an eternal swing back and forth just over a small center line that the capitalists determine.
Reformists have the greatest of intentions: to peacefully transition into a socialist society from a capitalist one. Fighting capitalism with reform is not a fair fight. It severely disadvantages the working class.
Revolutionary action is the worker equivalent to capitalist manipulation and is a necessary part of the toolbox.