Read two entries
Use this made-up harbour example: the morning low is 0.6 metres at 08:00 and the following high is 3.4 metres at 14:00. The numbers use the same reference level. Subtract the low from the high to get a tidal range of 2.8 metres.
Read the direction of change
At noon, you are between the listed low and high. In this simple example the general direction is up. The change needn’t be the same amount each hour. An exact height at noon would require more detail than those two entries give.
Ask a different question
What if you want to know when the water is moving fastest through a narrow harbour entrance? You would look for a local current prediction. Our two heights do not give speed or the time of slack water. The table is invented to practise interpreting a prediction.
For the underlying idea, read what drives the tides. Explore more routes in the water cycle hub.
Read the source: NOAA Ocean Service · What is a current survey?.