In general, you’ll need reservations on most trains you'll take in France, Italy and Spain. Plus most international trains that touch those countries. Then there’s a large chunk of Europe where reservations aren’t necessary, then further east they are often required but tend to be much cheaper.

There’s a good summary here, which includes a map of which countries are good/ bad/ OK for Eurail, and very detailed advice on how to make reservations, and how to decide whether a pass makes sense for you.:

https://www.seat61.com/how-to-use-a-eurail-pass.htm

Just to add : reservations are mandatory for all night trains I know. Expect to pay 30-50€ for a couchette, a bit more for a sleeper.

Night trains (especially Nightjets in Western and Central Europe) tend to sell out quickly so book them soon. :)

Let us know for any other questions, always add the date, time and route (it's easier).

I can really recommend seat61's website too ! This page in particular :

https://www.seat61.com/interrail-and-eurail-reservations.htm