Being barred means a casino has banned you. At a land-based venue you are refused entry; online, your account is closed and new registrations from the same person are blocked.
The reasons are similar in both worlds. Physical casinos bar people for cheating, such as capping bets or marking cards, for advantage play the house will not accept, like card counting, and for aggressive or disruptive behaviour. Online casinos close accounts for breaches of their terms: duplicate accounts, underage registration, payment fraud or systematic bonus abuse.
Barring is different from self-exclusion, which players request themselves to limit their gambling. Operators must enforce both lists reliably. That relies on KYC checks at registration, device and payment matching, and monitoring for irregular playing patterns, all managed from the back office.