In simple terms it's the constant changes we see in temperature, humidity, barometric pressure, precipitation, and so on. All the experts say these things effect how fish behave. While they do have their effects, it's foolish to plan your fishing trips by them. Obviously if the weather creates an unsafe situation, it would be better to wait for more favorable conditions. But remember, the conditions you experience above water are far different than what the fish experience below the surface.
Weather can alter a fish's environment in several ways.
Wind can stir up water and add oxygen. Precipitation can wash food into the water, or cloud the water with silt or other run off materials. Clouds can make it darker so the fish will move shallower. And those are just of few of many examples. I say ignore it because you still need to consider many of the same things no matter what the conditions or time of year.