You have to check some generic ldap info regarding all ldap related values, i.e http://en.wikipedia.org/wiki/LDAP
LDAP service is usually represented by some external service, i.e Sun Directory Server. It is mostly used for storing user related and environment/network related data. You can think of it as a fast non relational DB for storing user and network specific data.
Answering your question on requirements for ldap service usage with Sun webserver 6.1 - you need to have an ldap server installed and configured in your network.
Check the following docs related to ldap service configuration with Sun webserver 6.1:
I have a brand new installation, and receive the same information cannot bind message. I have entered the correct ldap information for at least two different ldap servers and this is the only error message I receive. I know the ldap information is correct because I use the same ldap for another sunone webserver installed on an aix box, and I can connect to the ldap server using JXplorer