Frak is an expletive analogous to the English word "[Wikipedia:fuck|fuck]".
It is used much like its English counterpart, i.e. "Frakkin' rats."/"Fuckin' rats."
It is a multipurpose and adaptable term, for it is capable of being used as a term describing the act of sex (ex: Tyrol's just fraked Sharon!) or can be used to denote one's hatred of something (ex: Frakin' Cylons).