A roller coaster is an amusement park ride that goes up and down inclines on a track. The term is often used analogously to describe things that go up and down. Do an image search to see some roller coasters and the analogy should be clear.

I’ll have a meal with a main course followed by a dessert. Eric writes about something with highs followed by lows. Because the clauses are long, the comma is needed for clarity. You could possibly punctuate it with a colon, or replace ‘with’ with something like ‘which has’. The original is much better though. The meaning is immediately clear.

WITH in this situation is probably best described as a preposition ... and here it gets tricky for me... "indicating that something is followed by something else!" ! With the warmer days now followed by an evening chill, I feel autumn coming. With the success of my presentation followed by my failure in the exam, I feel I'm on life's rollercoaster once again. I think it may be useful to think of it as an expression. It indicates a connection, in this case of one thing causing or following another. With all this confusion over WITH, followed by my even more confusing explanation, we're pretty mixed up, aren't we!