Heavily depends on the route. Most European trains have optional reservations or no reservations at all, simply add the train to the app before boarding (in order to get a QR-code, your ticket) and sit wherever you want. However optional reservations are often recommended in summer, especially in Germany and between capitals/big cities, they do not cost much (3€ through OBB).

On the other hand some trains have mandatory reservations, mainly high-speed ones and in France, Spain, Italy and some cross-border routes. Book well in advance as those trains can sell out (some more than others, like Eurostar and Thalys as there’s a quota for passholders).

If you don’t have a reservation for those, either you’re not able to get to the platform or you’ll get a big fine while on the train (it’s like travelling without a ticket).

Have a look at this link : https://www.seat61.com/interrail-and-eurail-reservations.htm

Do not hesitate for questions.

As you seem to live in Spain, railway links to France are very limited :

• TGV Barcelona - France : 25-35€ seat reservation. Book ASAP as that train is very popular.
• reservation-free regional trains via Latour-de-Carol (Pyrénées) or Cerbère (Mediterranean). From there either further regional trains or night train to Paris
• Euskotren between San Sebastian/Irun and Hendaye : not included but tickets cost 3-4€ (route not always shown on planners)
• AVE between Barcelona and Lyon (from 13th July)/Marseille (from 28th July) : should cost 10€ but not 100% sure if they accept passes. Ask at a RENFE ticket station (feedback very much appreciated).

Passes are valid according to their conditions but it's unclear how to get reservations if you're not in Spain.

That really depends on where you will be travelling. 

Which countries are you planning to visit?