Al giorno = "a day" / "per day" (it's used for repeated events)
"mangio una banana due volte al giorno" ("I eat a banana twice a day", literally "twice at a day")
You can also use "a + another time interval", e.g. "vado in piscina tre volte alla settimana" ("I go to the swimming pool three times a week")
"Il giorno" = 1) day time (as opposed to "la notte"); 2) a 24-hour interval; 3) a date ("il giorno della scoperta dell'America è il 12 ottobre" = "the day of the discovery of America is October 12")
"Ogni giorno" = every day. You use it when an event recurs once per day. Compare:
"Mangio una banana ogni giorno" ("I eat a banana every day") to "Mangio una banana due volte al giorno" ("I eat a banana twice per day")
Instead of saying "ogni giorno", you can also say "tutti i giorni" (literally "all the days").
I hope this helps! :)