Wats the difference between free and busy in the calendar?

Generally those settings would be use in a work situation where people might be looking to schedule meetings with multiple people. For example, I enter my work schedule into my calendar as "free" because I can still schedule work meetings during those times. However, if I schedule a conference call, I would schedule that as "busy" because I can't schedule anything else during that time. I might schedule an all day event such as a birthday as free as it has not bearing on my scheduling. However, I might schedule an all day/ multi-day event such as a days off or vacation as busy because I don't anyone thinking I'm available for work stuff.

Honestly, unless you're using it in collaboration with other people, it probably isn't going to be terribly useful to you.

Best of luck.