Entendiendo las reservas

First of all. Don’t activate your pass and travel days in advance. You should immediately deactivate your travel days and the pass. Please read more about that below in my next reply.

As soon as you know when you want to travel it is wise to make reservations for the trains with mandatory reservations, like the Eurostar and the TGV between Spain and France.

Here is some useful information from the experienced travellers in the Community regarding both planning, reservations and activation of pass and travel days. 

 Planning

The rail planner is normally not up to date, as it only is updated once a month, so to be sure of the time table you better check the timetable and availability on the websites of the national railways. The bigger national railways, like DB (Germany) SBB (Switzerland) and ÖBB (Austria) cover several countries. 

 Reservations 

There is a limited number of pass holder seats on the Eurostar and on popular departures and during high season those sell out weeks, and sometimes months in advance.

The best place to see the availability of passholder seats and make reservations on the Eurostar is 

https://www.b-europe.com/EN/Booking/Pass#TravelWish

Press "later trains" and eventually you will find the next available connection. 

If you have a mobile pass you need to generate a Pass Cover Number in order to make the reservation at b-europe. You do that here in the PCN generator:

https://community.eurail.com/news-and-announcements-39/pass-cover-number-generator-is-live-5653

Just to help understand the previous posts here is a basic introduction to your pass. (Apologies if too basic).
 

The mobile Global pass has 4 key functions - The IR train planner, A link to the IR reservation service, The Trip and your actual pass.

The first three are simply a set of tools to plan and prepare for your journey and the last one is the section that eventually becomes your ticket to travel.

The planner and the reservation tools are also available for many other sources and have no direct link to your pass. (Think of looking for a hotel and then reserving a room with one of several agencies)

When you add your pass to the app you have to immediately create a dedicated trip that you can name whatever you want. You only have one trip for the whole pass, even if you do a number of separate excursions with that pass.

The Trip is effectively your diary or calendar of trains you plan to travel on,but at this stage it is simply a wish list. Like all diaries you can add and subtract planned events at any time right up the last second.

Now we come to your pass. This sits dormant until you activate it before you board your first train, normally the morning of your first travel. This minimises any risk of wasting your valuable travel days.

Activating your pass creates a travel day for that day, but until you add the first train you do not have a valid ticket to travel. You add the trains you intend to travel on during the day by activating the slider against the train details in your trip. If for any reason you then do not board that train, but another on the same day you remove the train from your pass by the same slider, add the details of the new train and operate the slider for the new train. You now have a valid ticket (The QR code + the text below detailing the train(s) for that day.
Future travel days are created the same way, but again the advice is only do this on the actual day as you cannot cancel a created travel day after 2359 CET the previous day.

Reservations are always totally separate to your pass and these are covered well in earlier posts.

Una duda más , por ejemplo yo desde Barcelona a Paris tengo planificado viajar el 5 de mayo , con requerimientos de reserva. El tren que me sale ahí es TGV 9704, así que lo busque por la página de reserva y me sale 118 euros. 
Asi es como funciona el pase , no asegura el asiento , ante esto igual hay que comprar el pasaje en tren y desembolsarme esos euros , lo mismo con el de Paris a Londres , el pasaje del tren me sale cerca de 70 euros 

sin embargo en los viajes de Bruselas - Brujas - Gante - Amsterdam no me sale reserva requerida, es decir ahí no tendría que pagar ningún adicional y sólo subirme a un tren y mostrar el pase ? 
quedare atento a comentarios , muchas gracias 

118 EUR is the price of normal ticket, not just the seat reservation. The seat reservation cost from Barcelona to Paris should be about 30 EUR or less. The reservation fee for the Eurostar is also around 30 EUR. The link to make seat reservations for the Eurostar is in one of my replies above. 

For the trains between Brussels - Bruges - Ghent - Amsterdam you don't need any reservations. You just add the journey to your Trip in the pass. Just before boarding the train you connect the journey to your pass and create the QR code.