A route is a lot like car directions, but airways/jetways and fixes/waypoints/navaids in the sky are used instead of highways, exits, and mile markers. In the case of aircraft flying IFR (under instrument flight rules, which all US airlines do when carrying passengers), a filed IFR route is required prior to the instrument flight because that block of moving airspace for that expected time will be protected/reserved for that aircraft by ATC. The known route is also used for planning purposes by ATC to understand the volume of traffic going into or out of the airport at any given time.
As a tip, here are a few rules to help you understand the route:
1) If there are
five characters in the name of the point in the route (DUNKS), it is most likely an intersection name between two or more airways in the sky. Think of these as entrance or exit ramps of the airways. Sometimes they might actually be used as an intersection, whereby the aircraft turns at that point to pick up an new airway.
2) If there are
three characters in the point (PMM or OBK, etc), this is a navigation aid (called a VOR), or a starting/terminating point for that particular airway.
3) If the point
starts with either V or J and is followed by numbers (J70), this is the name of the airway. V airways are called VICTOR airways, or low IFR altitude airways (airways below 18,000 feet in the US) and J airways are called Jetways and are high altitude airways (airways starting at 18,000 feet and up).
4) If the point
ends in a number and finishes out the route (MOD3), this is called an STAR, or Standard Terminal Arrival Route. A STAR is a "prepackaged" arrival route, or mini-route, that contains its own series of points to take the aircraft from the arrival point (in this case most likely OAL in your route) to a point near the airport where ATC will vector the aircraft for an instrument approach. Not all routes will have STARs, but most IFR routes to the largest airports will.
Here is a screenshot of a high altitude IFR chart with the first few points along the way annotated. You can make out lower Michigan, Illinois, and Lake Michigan on the chart:
In the case of your flight's route, the aircraft will depart Detroit and be vectored by ATC out of the airport (away from other traffic) and to the first point along its filed route (DUNKS). Once the pilots are given "DIRECT DUNKS" by ATC, they will be told "RESUME OWN NAVIGATION." This tells the pilot to go ahead and fly the rest of your filed route.
Note the black lines on the chart, as they are the airways/jetways. The navigation aids, or in this case PMM and OBK on your route, actually send out radio waves that define the airway/jetway and many aircraft use these radio stations to navigate between these points.
You can
browse low altitude and high altitude IFR charts for free here. Airline routes will almost always utilize high altitude routes, since the en route portion of the flight is conducted well above 18,000 feet.
