This was a third party utility that the Gatwick Aviation Society produce. Amongst other things it could monitor the BaseStation database for new aircraft records and update them with information downloaded from their website.
The messages that aircraft transmit only identify the aircraft by the ICAO24 code. They do not contain information about the aircraft such as the registration, the operator of the aircraft, the name of the manufacturer of the aircraft and so on. It is these ancillary bits of information that SBSPopulate updated the database with.
SBSPopulate has been superseded by a pair of programs from GAS called Active Director and Active Director Lite. The latter is currently free, the former needs to be paid for. Details of all these programs can be found on the GAS web-site.