What is the Default Context in Authentication Configuration?

This is the information that is needed to find the users in the directory for Active Directory Domain Servers for Windows servers.

Example:  CN=WebCalAdmin,OU=Security Groups,OU=Groups,DC=hasm,DC=com

CN stands for common name and is used to identify users, groups and computers in your LDAP string.

OU stands for organizational unit and is used to identify OU's that acts as containers inside which other objects can live - e.g. the divisions, departments, etc. of your organization used to create a hierarchy and a structure.

So these two are totally different - OU's are used to build up a tree of nested containers for objects like users, groups, computers.