This is the 745th most frequent Bulgarian word.
крайно
Extremely; overly; at the end.
Here, "крайно" is used as an adverb meaning 'extremely' to emphasize the degree of importance.
Това е крайно важно.
This is extremely important.
In this context, "крайно" is used as an adjective to describe the position, meaning 'outermost' or 'extreme'.
На пътя има крайно дясна лента.
There is an outermost right lane on the road.
Here, "крайно" again functions as an adverb but in the sense of describing the intensity of the behavior as 'extreme' or 'radical'.