Bitcoin was created in 2008. It is a digital currency. You could regard it as digital cash; a so-called cryptocurrency. Bitcoins are bought on the internet. You will need a bitcoin wallet for it. You always buy bitcoins against the then valid exchange rate.
Bitcoins are primarily used by tech firms and the gaming industry, but also in online retail and even by travel organizations.
You don't need a bitcoin wallet to accept bitcoin payments via Mollie. Your consumer pays in bitcoins, but your bank account is conveniently credited in euros.