A profile describes your PC and its various components; CPU, GPU, Hard Drive(s) and others. You can create multiple profiles for the same PC, which enables you to track and compare performance between profiles with the same hardware but with different configuration (eg: baseline performance vs overclocking).
PC Baseline uses the OpenHardware reporting tool to gather information about your configuration. Instructions on how to install and configure the Open Hardware Monitor tool can be found here.
Once the tool is installed, you can create a "report file" by maximising the Open Hardware Monitor window, then clicking File - Save Report. This will create a text file which contains all the information on your PC - CPU, GPU, storage devices, main board and more. This data will be uploaded into the database to log all available metrics that can be tracked by the tool and therefore, what performance metrics can be analysed by PC Baseline.