Water weight is exactly that. More water in your body. Consuming a lot of salt or sugar can cause your body to retain extra water.

The human body is about 80% water. And an adult male can have their body weight fluctuate up to 5% completely safely just based on their water weight. If you eat a big salty meal and drink a lot of water, you'll gain a lot of weight pretty immediately. But as the salt works it's way out of your system, so too will the excess water. And you'll drop a lot of weight. By mildly dehydrating yourself you can lose a lot of weight.

Personally my weight can fluctuate by about 10 lbs. Just based on how hydrated I am.