An Earth Shattering Fart

Okay so today I read "A Sound of Thunder" I didn't really understand what was going on in the beginning of the story. But I kinda got what was going on later on in the story I got to the part of when Travis is explaining why they don't belong in the past. Also how they can effect the whole animal species by just killing one little mice. However can someone please care to explain what was going on in the beginning of the story please.