Have you already looked at the website, e.g. at What is Interrail?

The rail planner app shows the minimum reservation fees on top of your pass. That does not mean they're mandatory and that does not mean they're available.

You're from the UK, so you could go to Czechia via Brussels. You would then only need a reservation for Eurostar (30€). Most other day trains only have optional reservations.

Thanks, I’m trying to understand everything on the website.

so I need to buy a pass and then book the tickets as shown in rail planner and hope they are available?

If you buy a pass, then you have a ticket.

For some trains, a reservation is mandatory; for many others a reservation is optional or not available.

Reservations can be booked in many places: websites, by phone, at ticket offices. The rail planner does not do book reservations but can forward you to some of the websites where reservations can be booked, including Interrail's own website.

Thanks. I'm beginning to understand the process.

@suejenner  Will you just travel within Czech Republic? Ceske Drahy (Czech Rail) have a own “summer ticket” that is quite cheap compared to Interrail https://www.cd.cz/en/typy-jizdenek/vnitrostatni-jizdenky/-35199/

I'm travelling from UK to Czech Republic

Through Germany you could use the 9 EUR ticket if you are travelling during the summer. The ticket is not valid on high speed trains and IC trains.

https://www.bahn.com/en/offers/regional/9-euro-ticket-en

Thanks,that's very interesting, makes a cheap trip 

If you are planning to take the Eurostar across the channel you should book that ASAP as the cheapest tickets sell out quickly. 

This 9€ is indeed very cheap-and valid till end of month, but ONLY=ONLY valid on slower regional trains=takes much longer and many more changes.  And ONLY IN Deutschland=pay for the tickets till and from there. If you are already even scared to go on the continent, probably not the best choice to jump in.

The tipical 1st time tourist to CZ hardly uses trains (and once you´ve seen some, will have grave doubts about the assumed climate-benefit, stinky old diesels from communist times), will stay a few days in Prahy and maybe visit also Cesky Krumlov and or Karlovy Vary or Marianske Lazne (spa towns). Its FAR easier and quicker to get to there by BUS-there are several, RegioJet is one of the best. Plus that railfares in CZ are quite cheap-and even 50% off for student<26 with ISIC or senior>65, so that a pass is not a good buy.

A pass will have its benefits if you also plan to do some stops en-route, f.e. Wien=Vienna or Berlin.