What is "water weight", exactly?

SolidGoaled

I lost 10 lbs last week - yes, 10!! Now, I am very overweight, and also am just starting out (after a long 3 month relapse) so I ate somewhere between 1200 and 1400 cals each day, and ate the exercise calories, too. I know that someone is going to say "that is just water weight" so my question is, what is water weight, and where does it come from, and does this water weight take up space like fat does, because my waist FEELS 10 lbs lighter - my face is also different looking to me.

Now, I DO NOT in any way expect 10 lbs per week - the loss has already slowed wayyyy down - I just wonder - if that initial 10 lbs is water, where does the water come from, and is it water that my body can do without?

Thanks!