The IBM Security Directory Integrator is an Enterprise Application Integration (read about EAI at Wikipedia). It is a middleware working as interface platform to exchange data between different systems quick and dirty. Most IBM Security Identity Manager Adapters for target system are based on TDI. TDI is a valuable and very powerful software with lots out of the box adapters for several system like databases, LDAP directories, SAP RFC, REST, File access etc.. TDI is a fast solution to connect two or many data sources to each other. It has the ability as well to integrate native Java code directly. The Downside, like in every software which has a high human readable abstraction of technology, very specific requirements can get difficult to solve and may result in large and complex TDI Assembly Lines - like the programs are called in TDI.
TDI is integrated in the licenses of some IBM Security products like the Access- and Identity Manager and therefore free of charge in conjunction with these products. However TDI can be used as standalone product as well.