* "Heure" is the singular form of the noun, meaning "hour". It changes to "heures" when referring to multiple hours, like "two hours" or "three hours".
* "Il est une heure" literally translates to "it is one hour". In this case, it's not referring to multiple hours, but rather to a specific time: 1:00 o'clock.
Therefore, the singular form "heure" is used because we are referring to a specific time unit, not multiple units.
This is similar to English phrases like "It's one o'clock" or "It's quarter past one". We don't say "It's one hours" because we're not talking about a quantity of hours.