EPP (Extensible Provisioning Protocol) is the protocol registrars use to talk to registries, and its status values appear in every lookup. The prefix tells you who set the flag: "client" codes are set by the registrar, usually at the owner's request, and "server" codes are set by the registry.
The common ones are harmless or protective. "ok" means no restrictions. clientTransferProhibited, clientDeleteProhibited and clientUpdateProhibited are the normal registrar lock that prevents unauthorised changes; most well-managed domains show them. The server versions of the same locks indicate a registry-level lock, often a paid high-security service.
The ones to pay attention to: clientHold and serverHold remove the domain from the DNS, so the site and mail stop working; they are used for non-payment, failed contact verification, legal disputes or abuse. autoRenewPeriod, redemptionPeriod and pendingDelete mark the stages after expiry. pendingTransfer means a transfer to another registrar is in progress. RDAP shows the same states written as words, for example "client transfer prohibited".
Example
Domain Status: clientTransferProhibited
Domain Status: clientDeleteProhibited
Domain Status: serverHold