More than I can explain you will find on the site of the Man in Seat Sixty-One.
Eurail, as well as other rail passes in Europe, have passes for a selection of days in a longer period as well as passes that are valid over a period.
And they have passes for a selection of countries (starting with just one) as well as passes for the whole of the network.
You said you plan to travel in the Netherlands, Belgium, France, Germany and Czech Republic. While this is 5 countries, Eurail combines BElgium, NEtherlands, and LUXembourg into one 'country' ("Benelux") for the purposes of the pass. So a 4 country pass will do. You can find the explanation on this page of the Eurail site.
Within the validity of the pass you can travel as much as you like and in any order.
As you are looking at a 5-days in two months pass, you need to know that you can use just those 5 days, and those days do have to fall within the two month period the pass is valid for. The other 55 days (roughly) you can not use the pass. If you want to travel by train more than 5 days you will have to buy tickets for those day or buy a different pass, which will cost more.
With one exception, if you want to use an overnight train, you can sometimes start the travel on the evening before the day the pass is validated.
While the pass pays for your tickets, you will still need to pay for reservations and at time surcharges. For some trains the surcharge for a pass holder can be as much as a ticket bought well in advance. And when traveling in summer and in weekends round the year as well as peak hour trains that commuters might want to use you need to make reservations well in advance.
So, unless you want to use that pass 5 times, like to travel from the airport to your first country, from there to the second and only use the pass for long travels on trains that do not require reservations, I would buy point to point tickets as far in advance as possible. Even two days ahead of time you can get fair reductions on many connections, if you are flexible with the time of day you want to travel.
And there are many trains connecting the countries you mention, often one every two hours and sometimes even every hour. Within the countries it is not unheard of having hourly direct trains and additional trains on the alternative half hour.
Train tickets within countries are mostly cheaper than international tickets. Travel all the way across the Netherlands is always cheaper than 1/5 of the rail pass.
The site of the Man in Seat Sixty-One has a kind of tool for working out whether the pass is worth it for you.
A good site for time tables everywhere in Europe as well as prices for trains within Germany, and often across the borders, is the German rail planner.