Tides
It's the height above the lowest tides,but you can still get negitives like say(-10cm).It's called chart datum,the depths on maritime charts are measured from that level.Tide range is measured from that level up.
it's measured on a scale so you can tell how low the low will be etc. Tide heights change all the time depending on the moon / sun rotational cycle / gravity etc.
when they say what the tide is i don't get the part where they say what it will be in cm like low tide 12:50 at (73cm)
i don't get what it means.