Basic facts:

1 litre of water weighs 1 kg (1.000 grammes)

1 litre of alcohol weighs 0,8 kg (800 grammes)

1 litre dissolved sugar weighs 1,59 kg

When the must begins fermentation the sugar content is high.
The hydrometer reading is then about 1080 degrees.
As the sugar ferments to alcohol the reading gets lower and lower.

Measure the gravity of the must at the start of fermentation.

Refer to the table "When the wine has fermented out" to see
what the hydrometer reading should be.

Measure the wine during fermentation.

By seeing where the reading stands between the start value and
final value lies you can see how far fermentation has progressed.