Railroading is the colourful metaphor used to refer to the act of a GM enforcing a predetermined outcome of an encounter or other plot element on the players, no matter what they try to do. It's generally spoken of in negative terms, as a criticism of a GM who can't handle the players doing unexpected things to his plot and whatever planned encounters or places have been created.

But in fact, some level of railroading is almost necessary for most games. Good GMs can "wing it" to some extent, but that taxes the mental processes necessary for on-the-spot creativity and it is hard to sustain for long with any degree of quality. So most GMs use tricks that give the players the illusion of doing whatever they want, while really placing the planned obstacles in their path at convenient spots so they can deal with them. With a bit of flexibility in the precise arrangement or sequence of the planned events, this actually provides a lot of freedom and to most players this is a satisfying way to run the consensual gaming experience.

So the lesson to be learnt from all of this is that it's of prime importance to tell your players that what they're doing makes a difference. Even if it doesn't.

Transcript

Pete: Well then. Anakin—
Anakin: I can hardly wait for this.
Pete: Multiple construction arms swing your way, tipped with sputtering arc welders, while four of the creatures fire at you.
Anakin: All right. Is there any point to rolling?
Pete: Of course there is! I'm not railroading you here. If you know the rules this will be easy.
Anakin: What do I need to roll then?
[SFX]: Ga-bloop!
Pete: I need to know what you're doing.
Anakin: Fine. I Force Push those hooks at the creatures and dodge the arms.
Anakin: Then I Push these gadgets at the other creatures and jump to a safe spot.
[SFX]: Ka-paf!
[SFX]: Whooom!
Pete: Hmmm.
Anakin: 13.
Pete: Not enough. You get knocked down and <roll>...
[SFX]: Smak!
Pete: The machinery welds a steel plate over your arm, immobilising you.
Anakin: Great.
[SFX]: zzzt... zzzt...
Pete: Did I mention the guillotine blades?