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.