A security module serves as a medium between the Directory Server and the TLS layer. The module stores the keys and certificates used for encryption and decryption. The standard which defines these modules is Public Key Cryptography Standard (PKCS) #11, so these modules are PKCS#11 modules.
By default, Directory Server uses built-in security databases,
cert9.db, to store the keys and certificates used by the servers.
It is also possible to use external security devices to store Directory Server certificates and keys. For Directory Server to use an external PKCS#11 module, the module's drivers must be installed in Directory Server.
For more information, consult the documentation for your hardware security module.