Train eurorail pass

Depends on what country and what type of train. If it is a high speed train in France or Italy, yes you have to reserve and you must be on that train that is listed on the ticket. Regional trains you don't need reservations and you can get on and off (assuming you are not on the last train of the day).
Read all of the gory details at www.seat61.com, specifically the section on rail passes.
By the way, fast trains will generally have a snack/restaurant car, and you can bring any food or drink with you and picnic at your seat. Many cars have tables.

As for Germany, unless you ride on of the rare ICE Sprinters, reservations are never required. If you are on a rail pass, you can basically get on any train (except the Sprinter), any time. If you want a reservation, a German Rail reservation is good for one train and one connecting train, so it you can commit to the second train, you should be able to get reservations for the first train, and a second train after a specified stop, but if you are unsure about which second train you want to take, you get the second reservation separately, once you are sure.

You certainly need to read the terms and conditions of use of whatever pass and, as already advised, the section in the seat61 website on rail passes before any commitment to any pass.

If you need a reservation, and if you want to get off a train and get on a different one later in the day, then that requires 2 reservations. But that is two if's, you may not need a reservation in the first place. What most people do is bring their own food on board, and eat that while the train is moving.

Don't forget that Eurorail is not a railway company, they do not own or operate any trains. They are a reseller of tickets and passes, which are valid on many different railway company's trains.
These railway companies, mostly national but some local, all have different rules, including about which trains do or do not require reservations, and whether Eurorail passes are or are not accepted.
This is all explained on the excellent http://www.seat61.com/ website, especially his page on passes: http://www.seat61.com/Railpass-and-Eurail-pass-guide.htm
This should be compulsary reading for anybody before buying a ticket or pass.

If you've not already bought the pass then good for you. Point to point tickets will probably be cheaper and easier for you than a silly multi country pass.

See more at http://www.ricksteves.com/travel-tips/transportation/trains/reservations. The DB link at How to Look Up Train Schedules and Routes Online is the most comprehensive source to see which trips (or which legs of a connecting trip) are "Subject to compulsory reservation."