Anonymous bind is a Bind Request
using Simple Authentication
with a zero-length bind DN
and/or a zero-length password
Anonymous bind may be used to destroy any previous authentication performed on a connection and return it to an unauthenticated state.
Note that there is an ANONYMOUS SASL Mechanism that has the same effect, but in general the term "Anonymous bind" refers to the simple bind Request with no DN and/or no password.
Some LDAP Servers may be configured to NOT permit Anonymous bind Bind Requests.
There might be more information for this subject on one of the following: