Eckels is warned several times during this story.  I'm not 100% sure which warning the question is referring too.  It doesn't really matter though.  Each warning to Eckels foreshadows the same thing.  They foreshadow that Eckels is going to get scared and ruin the present/future.  The first warning to Eckels is that the trip is dangerous.  It's so dangerous, he might not come back alive. 

"Does this safari guarantee I come back alive?"

"We guarantee nothing," said the official, "except the dinosaurs."

The second warning to Eckels is more official.  The warning this time is a signed release document.  The document is meant to force customers into making sure that they are fully committed to the dinosaur hunt.  People who are not mentally ready tend to panic.  

"Sign this release. Anything happens to you, we're not responsible. Those dinosaurs are hungry."

Eckels flushed angrily. "Trying to scare me!"

"Frankly, yes. We don't want anyone going who'll panic at the first shot."

The warning foreshadows exactly what happens to Eckels.  He panics so badly that he can't control where he is walking.  

The third warning deals with where Eckels is going to end up walking.  The safari customers are specifically told about staying on the path.  It is strongly emphasized to not leave the path.  Leaving the path might lead to irrevocable changes in the timeline.  

"Its purpose is to keep you from touching this world of the past in any way. Stay on the Path. Don't go off it. I repeat. Don't go off. For any reason!"

Those three warnings all foreshadow Eckels's disastrous encounter with the dinosaur.  Eckels realizes that the dinosaur is the most dangerous animal that he has ever faced.  Next, he panics.  His panic then leads him to step off of the path and kill a butterfly, which causes an entire societal and governmental change in the present.