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