The tides are measured in metres on the tables. A reading of 6.24 means the tide is 6.24 metres high, and is therefore the High Tide reading. A reading of 1.24 means the tide is 1.24 metres high, and is therefore the Low Tide reading.
Tides are measured from a level called 'Chart Datum'.
Chart Datum is approximately the level of the lowest astronomical tide (LAT). This is the lowest level that the tide falls to under the influence of the pull of the moon, sun and planets.