BEP-20 is the token standard of BNB Chain (formerly Binance Smart Chain). Technically it is a copy of the Ethereum chain that uses fewer validators and is therefore considerably cheaper and faster — a transfer costs cents and usually confirms in under a minute.
Why casinos offer it
A cheap network means a small deposit makes sense. Where sending five euros costs a euro on the BTC chain, on BEP-20 it costs a couple of cents. That is exactly why it is the default recommended option in many cashiers for stablecoins and wrapped coins.
The risk: an identical address format
A BEP-20 address starts with 0x exactly like an ERC-20 one. They cannot be told apart by eye. If you send ERC-20 USDT to a BEP-20 address or the other way round, the money is lost practically every time. The only protection is to choose the network in the cashier before copying the address and to check that the wallet's network matches.
Wrapped coins
"BTCB" moving on BEP-20 is not bitcoin but its wrapped equivalent. A casino expecting real bitcoin will not credit a BTCB transfer. That is a separate trap on top of the network choice — see the Bitcoin casino list and the payments guide.
In summary
Fast and cheap, if you know exactly what you are doing. For a beginner TRC-20 is the safer choice, because an address starting with T cannot be confused with any other network.