What exactly is "water weight"

Often when people start on a weight loss/diet plan, they see a quick loss and this is motivating for them - this isn't a bad thing, obviously.
However, the first large amount of weight is partially due to your body mobilising stored glycogen from your liver in order to increase your blood sugar to provide energy for your body; liver glycogen stores are the first your body will use - they are the stores you use when you don't eat for periods of time (such as overnight). (Glycogen is one way your body stores excess blood sugar to clarify).

When this process occurs, your body move around about 300mL (can be more or less depending on the person) of water from the inside of your cells to cause the glycogen to go into your blood stream. This is then filtered out through your kidneys and then you pee it out. However, this water will eventually be replaced by water from what you drink/eat and your cells will fill back up and hence you will put the weight that you have 'lost' back on as you were not losing body fat, just the fluid from the inside of your body cells.

A pretty similar process occurs with diuretics or things that dehydrate you such as alcohol. Your body will filter your blood more, leading to more urine passing, so your body will take fluid from the inside of your cells to replace the fluid in your blood - Hence you look smaller and weigh less when you are dehydrated but you go back to normal and weigh more when you hydrate.

I hope that made sense and wasn't too technical.

I don't have exact sources, but I am studying medicine - if anyone feels they can explain it better/I've got anything wrong, feel free to change it