According to Wikipedia Lightweight Directory Access Protocol (LDAP) is an application protocol for accessing and maintaining distributed directory information services over an Internet Protocol (IP) network.
LDAP was originally intended to be a lightweight alternative protocol for accessing X.500 directory services through the simpler (and now widespread) TCP/IP protocol stack.
This model of directory access was borrowed from the DIXIE and Directory Assistance Service protocols.
When an web application is unable to properly sanitize the inputs provided by the user, it is possible for an attacker to alter the LDAP statements, this can raise serious security problems and can help the attacker to add/modify the LDAP tree.
LDAP injection is a bit similar to SQL Injection attacks.