It's the period of time that is considered when calculating the figure.
Overtime is set up with thresholds, effectively a chosen amount of hours that if reached and surpassed then counts as overtime time.
These are recorded over weekly periods or daily. If we look at daily, for example, an employee can have an 8-hour day standard. If they work 1 hour for 4 days and then 9 hours on the last day they will have qualified for 1 hour overtime on that last day as they surpassed the 8 hours in their working pattern. The Overtime doesn't take into consideration the several hours missed on the ays before. It's only concerned with it's threshold being met.
The threshold being daily means it looks at that one day and whether the 8 hours has been surpassed. In this case, the 9 hours meet it.
The offset for this employee is going to be negative. For 4 working days, they worked for 1 hour meaning 4 x 7 hours were not worked. That's minus 28 hours in offset for those 4 days. The employee then works an extra hour on the final day and ends the week with a - 27-hour offset but a 1-hour overtime amount.