A bitemporal cell is a two dimensional representation of an event.
We can think of the two dimensions as event time and transaction time. For example, let's say we want to record the information that between 9 AM and 11AM, the latest information was that a particular flight from New York to Chicago would take off at 8:35 PM and land at 11:15PM. Assume all times are in UTC for simplicity. Now, we have the transaction time as 9AM to 11AM. We have the event time or valid time as 8:35PM to 11:15PM. Now, let's say at 11AM something changes. Let us say this is because of the weather. We are no longer sure if this flight will even take off today. This is horrible for the airline company. Let's say at 2PM, the flight is rescheduled to take off at 9:15PM and land at 11:45PM. We have another bitemporal cell.
In summary, a bitemporal cell is a data structure which allows us to logically delete data without actually physically deleting any data. After all, there is money to be made!