What is the definition of kalus rosh?
What are the halachos of kalus rosh and where do they apply?
Shulchan Aruch (OC 151:1) lists three examples of kalus rosh, though doesn't define it explicitly:
בתי כנסיות ובתי מדרשות אין נוהגין בהם קלות ראש כגון שחוק והתול ושיחה בטילה
We don't practice kalus rosh, like mirth, jest, and wasteful conversation, in synagogues and study halls.
Mishna B'rura ad loc. clarifies "wasteful conversation":
That is: Even secular conversation for the purpose of sustenance — which is permitted outside — is forbidden in the synagogue, and specifically completely wasteful conversation, from which it is seemly to always refrain, and of which sin the Zohar (Vayakhel) greatly amplified the significance.
To make light of something important. See Rashi on the bottom of Megila28a. Kallus means that you make light of them. I am assuming he is learning the word 'rosh' is referring to the batei kneisios the gemara is discussing, with rosh meaning something important. Apparently it does not mean to be light headed!