A rate represents the raw exchange rate at a specific time for a rate pair. Rehive's global rates are updated every 5 minutes from Open Exchange Rates and the latest one is used when displaying values in the wallet app in a user’s set display currency. Rates are a snapshot of what a rate pair was worth at a specific time.
The Conversion Extension stores a history of rates between currencies - for the recent past, it stores the rates for every 5 minutes. After 7 days it only stores one rate per hour per day.