You need to study some transmission line theory. I'm feeling generous, normally with a question like this, I'd just post a link to your local university, instead here are the basics...
When you push an electron into a wire, another doesn't pop out of the other end instantly. There is a small delay.
The velocity of a microstrip structure on FR4 is *about* 180ps/inch.
So if your microstrip transmission line is 1 inch long, and you apply a 0-1V step at one end of the line at t=0s, you'll see that step appear at the other end at t=180ps.
You really need a book. It's a big subject.