First of all it is writte BUONGIORNO

buongiorno (or very rarely bongiorno, or the more common buon giorno)
is it for good day, or good morning
It is composed by buon(o) (from buono means good) and giorno (means day)

(with tones) buongiòrno bongiòrno buòn giòrno

It is used as well-wishing greeting, above all in the morning or during the morning (it's better to use it before 12:00)

f.i. "buongiorno a tutti!" "buongiorno mastra" "buongiorno amore"
good morning everybody! good morning teacher! good morning darling!