The note at the bottom is there because 軽佻浮薄 is a so-called "yojijukugo" or four-character compound, a phrase consisting of four kanji that don't make any grammatical sense (in Japanese at least) but symbolize some deeper saying or phrase. They don't necessarily make any sense to even Japanese readers if they haven't seen them before, hence the author's explanation.
TRULY IS FRIVOLOUS AND THOUGHTLESS!!!
Laughing when confronted with serious scenes...
Please sign it "To Hijiri Byakuren ♥"
Because I'm a fan of Akyuu-san!
BYAKURE—N!!
No problem!
What a fangirl.
*軽佻浮薄 (Keichoufuhaku)
"Frivolous and thoughtless".
Acting rashly and carelessly.
Thinking superficially and shifting your attention often.