@Noavocado there's lots of science for different stages. If you're new to this, your metabolism will be going through a major readjustment. It's habitually be using glucose for energy, and now it's using fat. Our bodies can do both (aren't we amazing creatures!!) but in the days of our ancestors they would eat mostly leaves, veg, nuts and fruit that grew above ground plus protein from animals and fish. They were in natural fat burning mode.
Then maybe once or twice a year, the tribe would find a big honeycomb and would gorge themselves on sweet sticky honey. Thank goodness for our insulin response, we'd have been wiped out instantly otherwise! For a few days they would lay around feeling lethargic and sleepy and then get back to the normal hunter gatherer life.
Modern life with easily accesible refined carbs and sugar has us in that honey-eating state most of the time. It's a big adjustment to change back to using fat for fuel (as a constant norm) so it can take a while for our bodies to get really efficient at doing it.
As @BIWI said, we lose weight quickly in the first couple of weeks as we use up our glycogen stores, and the water molecules associated with them. Then our wonderful amazing bodies are transforming to adapt to this way of eating.
As we keep on with this woe, we gradually lose fat. Fat doesn't exist in our bodies as a big amorphous blob, fat is contained within fat cells. As we lose fat, the amount of fat in some fat cells reduces and is replaced by water (the cell is still waiting for the cell to be filled up again with fat). I think... that water is more dense and therefore heavier than fat, so there may be a period when the cell is more water than fat, and therefore it will weigh microscopically more. Eventually the fat in a particular cell is all depleted, the cell collapses and the water is released. This results in a characteristic 'woosh' in downward weight, often accompanied by peeing like a donkey 
.
Of course the process doesn'e happen one cell at a time, but maybe that depiction helps to explain why weight loss is not linear, and also why we need WATER WATER WATER for the fat burning process. The actual biochemical process of using fat for energy requires water for the molecular reaction.
I hope I've managed to add a shematic which explain this fat cell theory...