Procedure linkage tables (PLTs) are structures used by the rtld to facilitate calls across shared object boundaries. For instance, calls to printf go through this indirect table. For details, see the generic and processor-specific SVR4 ABI reference manuals.
For dbx to handle step and next commands across PLTs, it has to keep track of the PLT table of each load object. The table information is acquired at the same time as the rtld handshake.