A Destination is the I2P In net address of the final endpoint you are trying to connect to (example: an I2P website, service, or Monero node). This can also include a local destination of which other peers need to connect to in order to make contact for communication (similar to how, in Clearnet, your IP address is given to a website when you connect so it knows where to send the information back to).
An I2P destination can be encoded into a Base32 address or Base64 address. Most users will only care about Base32 addresses or a
.i2p hostname while, internally, Kovri / I2P Address Book uses Base64 addresses. Ultimately, all Destinations in I2P are 516-byte (or longer) keys:
256-byte public key + 128-byte signing key + a null certificate = 516 bytes in Base64 representation
Note: certificates are not used now but, if they were, the keys would be longer.