Dehydration is when your body does not have enough water. This can cause many health problems, including dizziness, lightheadedness, headaches, and even seizures in some cases. You can also get dehydrated from sweating too much or being in the sun for a while without drinking enough water.
In severe cases, you may pass out due to extreme dehydration.

The best way to avoid dehydration is to keep drinking water, and to avoid drinking alcoholic beverages or sugary drinks.
If you are feeling dehydrated, you should try to drink some water and eat some salty foods. This will help you to feel better soon.

The amount of water you need depends on your body size and weight, normally one liter per 20kg of weight.
If someone has a more educated response, I will definitely stand corrected! 🙂