Railroading is often described as forcing the party through a story, but the reality is that it's more about robbing players of agency. RPGs are always bounded in some way, but players get to exercise agency within those boundaries, and that's where the gameplay emerges. Sometimes the outcome of a situation is set (Get in the fucking robot, Shinji), but the way they get to that outcome can be extremely enjoyable. Do you go through the window or the front door? Do you fight or sneak? Do you try charm or intimidation? Railroading is when the GM takes those decisions away from the player, which is anti-fun.