If you only use one dialogue database, you can skip this page.
Internally, every element in a dialogue database (actor, item, conversation, etc.) has an ID number. Within a single database, these IDs are usually unique for each type of asset. However, it's likely that an ID used in one database may also be used in another database. If you load both databases at runtime, the IDs will conflict. The Dialogue System won't know which asset you're referring to with a given ID.
To prevent this, use the Unique ID Tool. Select Window > Dialogue System > Tools > Unique ID Tool.
Simply add the databases for which you want to guarantee unique IDs, and then click Process.
If you tick Verbose Logging, the tool will log all ID number changes in the console.