A marker is a form of casino credit: a cheque or IOU a player signs to draw chips against an approved credit line so they can keep playing. The word can also mean the player who signs it. Not everyone can get one. Credit is usually reserved for VIP members and regulars who have built a relationship of trust with the casino.
Once credit is approved, the player asks for a marker from the staff at the table or at the cashier’s cage. Markers are not all the same, so check the terms first, above all how long you have to repay before the casino can collect the money. Credit play is mainly a land-based practice. Many online jurisdictions restrict or ban gambling on credit.