“Railroading” is often used overbroadly to the point of meaninglessness. My title here is supposed to be poking fun at that a little.
As a negative term “railroading” is supposed to refer to when you actively prevent the players from exploring or doing something they want to do, and that should work, because it doesn’t follow a set path you’ve planned out:
Example: you plan for a villain to taunt the characters, fight them briefly, and then escape. The players do better in the combat than you expected, and would have removed all his HP, so you bump the HP up, then on his turn have him cast dimension door to escape. When a character counterspells the dimension door, you give him counterspell to counter that counterspell. When a second counterspell happens, you have one of his mooks pull out a scroll of counterspell and use it… Because you need him to escape for your plot, so you force it.
That’s negative railroading. It’s much better to let them succeed, then alter your plans for future sessions accordingly. Maybe there’s more to the evil scheme than just this one guy, maybe they won the day and stopped the threat and the next arc is something else (and maybe you can re-use that dungeon you already planned out)
It’s perfectly okay, and in fact often a good idea, to plan out the broad strokes of your story in advance, you need to plan something and you can’t plan for everything - so having a good idea of what’s coming is usually the right move - it’s only bad if you refuse to let the story go to other places.