The Pearl Injector is a block added by RFTools and is a part of the RFTools Endergenic Power Generation System.
The Pearl Injector's function is to insert an Ender Pearl into the adjacent Endergenic Generator at the appropriate time. It is not configurable and the GUI contains only an inventory for storing Ender Pearls. The Pearl Injector is not operated directly. When it receives a Redstone pulse, it will insert an Ender Pearl into an adjacent Endergenic Generator.
For full details on placement and usage, see the guide on Endergenic Power Generation.