The time-slice is whatever you define, or whatever is specified to you.
In the case of stocks, it's usually the high, low, close for the day.
If you're mapping tides, and measure the same time every day for a 15 days, then you would take the high, low and close (last reading) to date and graph those 3 values. If you're graphing petrol prices, you can take readings every n periods and graph the h/l/c in the defined range. The point is in graphing the 3 measurements. The Low has to be the lowest value for the period, the High has to be the highest value for the period, and the close has to be somewhere >=Low AND <= High. It's a measure of agression for the period.
How you acquire the periodic measurements is up to you and your requirement.
Edit: The period simply defines which set of records to use to select H/L/C from.