The chart interval is the time scale of the chart that is chosen.
Each interval shows a point such as on a line chart which can be O pen, H igh, L ow or C lose or user chosen average. Often in formula you may see it as O/H/L/C.
It can be a bar where O pen is on the left, High and Low being the bar extremes and C lose being on the right. I mainly use bars as it often shows and helps in wave identification.
Candle charts generally have H and L being the wicks and the body O and C.