When you start Kovri for the first time (or if it's been offline for a long time), Kovri will need a list of peers to connect to so it can bootstrap into the I2P network. Kovri gets these peers from a special file stored on a reseed server. On this file are all the various pieces of information Kovri needs in order to connect with I2P peers.
Kovri has a list of hard-coded reseed servers available to fetch from. These servers securely serve an SU3 file (signed with a cryptographic signature) over Clearnet with HTTPS. This SU3 file contains information that's used to verify both the integrity of the file and its content.
Aside from the technical elements needed to verify and process the file, the file's main contents consist of a series of Router Info files which Kovri and I2P routers use to locate and communicate with other I2P peers. These peers are then stored into a Network Database.