For travelling between Amsterdam and Rotterdam by train, there is a mention of a supplement to pay for some of the "IC direct" services. Other trains do not mention the need to pay this supplement.
Why is this supplement system in place? Which are the affected services in general?
As a passenger who uses preloaded credit on my OV-chipkaart and rely on the tap-in-tap-out actions at the gates to pay my fares, how can I buy this supplement before boarding a service that requires it?
Why is this supplement system in place?
Because they can get more money from people who wish to pay for faster travel time.
Which are the affected services in general?
You must pay for the supplement if you use Intercity direct services between Schiphol and Rotterdam. These trains are marked with indicating of additional supplement in the Journey planner: https://www.ns.nl/en/journeyplanner/.
You do not have to pay for journeys between Amsterdam Centraal and Schiphol, or between Breda and Rotterdam Centraal, as these journeys do not include the high-speed line between Schiphol and Rotterdam.
how can I buy this supplement before boarding a service that requires it?
Quoting from https://www.ns.nl/en/season-tickets/campaigns/other/intercity-direct-supplement.html:
You can buy the Intercity direct supplement from the Supplement Pole at the departure platform, online as an e-ticket, from the NS Ticket Machine or from the service desk at the station.
You have the option to buy the supplement when you buy a ticket from the website or the app.
The supplement pole looks like this (https://commons.wikimedia.org/wiki/File:Intercity_Direct_Toeslagpaal.JPG):
In particular
To buy a one-way Intercity direct supplement, simply hold your OV-chipkaart, debit card or NS-Business Card against the card reader on the Supplement Pole. The supplement will be added to your card automatically.
When you reach your destination, simply check out at an NS card reader or gate. You do not need to check out at the Supplement Pole.
If you want to cancel the supplement, hold your card against the Supplement Pole card reader again within 30 seconds, and it will be removed from your card.
