This file is, effectively, your personal Certificate Authority. It is the list of all SSH server host public keys that you have determined are accurate. Each entry in known_hosts is one big line with three or more whitespace separated fields as follows:
a. One or more server names or IP Addresses, joined together by commas.
foo.com,107.180.00.00
b. The type of key.
ssh-rsa
c. The public key data itself encoded to stay within the ASCII range.
AAAAB3NzaC1yc2EAAAABIwAAAQEAuJfqSnraBz//Ux4j/hZpLv2eYUxNUgCk+9ClqoSgfcu4vXbWtUGSjo75UVQf+uguOeBnRLppJJ3mt0R5c/PPcawUGWfffk33t+biYcqra9xUcyfiGtO/Icko2L1J0EYTXM/8x8VK6UYFMfad2gltnZRa8Am50oHTXot1Df0RljUBxvh/UhmTJUrODpyrl2xY1OMWjM+S6uYCMNeSQGEpNfsWiCIStRnctMZSxiYJOLTSC4F2GF7B8pYFBn5rSwVHp17WCdO+4BZfwvH3HSSH8IWoyFhki+NlG912SEBJXcryvc0JPfAB9DTB4mRImjgrRT8vz5QeaCDrh8k4/A+U1fff
d. Any optional comment data.
Also!!
This thread might be of use for you:
https://security.stackexchange.com/a/20710