Bitcoin is a digital currency, created in 2008. You could think of it as digital cash; a so-called cryptocurrency. Bitcoins are bought on the internet against the current bitcoin exchange rate. To store your bitcoins you'll need a bitcoin wallet.
Bitcoins are primarily used by tech firms and the gaming industry, but there are also a variety of online retailers and travel organizations that accept bitcoin payments.
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.