This is an exclamatory expression which is used on many different occasions. In English, it can be translated, “Look!”, “Listen!”, “See?”, or just before saying something like “Here it is.” All these phrases are to catch someone’s attention. So it is used when someone you are talking to isn’t paying attention to you. For example, “Hora, Fujisan ga mieru” is “Look! We can see Mt. Fuji.” “Hora, basu ga kita” is “Here comes our bus.” Or “Hora itta desho” means “See? I told you so.” When you are about to give something, say a bowl of rice, you can say “Hora” like, “Here you are.” Lastly, it is very colloquial, so it is better that you do not use it with your boss, but use it among your friends.