What each value records
The open is the first price recorded for the period and the close is the last, under the conventions of the data series. The high is the greatest price recorded and the low is the smallest. The open and close must both lie within that range. Either can coincide with an extreme, so a bar can begin or end right at its high or low.
An OHLC bar uses a vertical line for the low-to-high range. A short tick on the left marks the open, and a tick on the right marks the close. A candlestick can show the same four values with a body in place of the two ticks. In the diagram, the tick on the right is higher than the tick on the left, so the close is above the open.
Compare like with like
Check the instrument and interval before comparing values. An hourly bar and a daily bar summarize different groups of observations. When comparing data from two charts, check which trading session each chart uses. A chart that includes observations from a broader session can show a different range for the same calendar date.
Common misunderstanding
High and low describe extremes, so their positions on the bar do not reveal which extreme came first. Two periods could have the same four values even if prices moved in a different order between the opening and closing observations. OHLC also omits the traded quantity. If the question is how much changed hands, look for a separately defined volume measure.