It’s an hour. Because the word "hour" begins with a silent "h" ("hour" and "our" have the same pronunciation), we use the article "an".
It’s an hour. The choice between "a" and "an" is determined by the initial sound of the word that follows, not how it is spelled. Because the word hour begins with a silent "h" ("hour" and "our" are pronounced in the same way), we use the article "an".
He was arrested an hour ago.
He was arrested a hour ago.
Remember, it's the sound, not the spelling, which is important.
The meeting will take an hour.
Getting an hour of sleep will always be better than not sleeping at all.
A partial list of words that also start with a silent "h", and therefore take the article "a":