In a nutshell, a reservation is a guaranteed seat on a specific train, assigned to you personally.
Whether you need a reservation or not depends on two things: the train you’re on and what type of ticket or rail pass you have.
Let’s start with the train you’re on. There are essentially three types that run in Europe, each with different requirements.
The first type of train is “Reservations Required.” This generally applies to high speed and overnight trains. Tickets sold for these trains always include a reservation. If you are traveling with a rail pass, you must purchase a seat reservation separately, at an additional cost. If you try to board these trains with your rail pass only, you will most likely get fined.