Kshira Sagar is a very big body of water that is said to exist in Hindu mythology. According to the mythology, it is basically an ocean of milk. This means that instead of normal water that we see in oceans and lakes, it is full of milk, kind of like a giant pool of creamy white liquid.
The story behind Kshira Sagar is that a very long time ago, the gods and demons worked together to churn the ocean using a big mountain, called Mount Mandara, and a long serpent, called Vasuki. This was done in order to get treasures from the ocean that would make them very powerful.
So they churned and churned and churned, until the ocean turned into butter. They then used the butter to make ghee or clarified butter, and from the clarified butter, they got amrita, a very powerful drink that could make them immortal.
When the gods finally got the amrita, the demons tried to get it too, but Vishnu, one of the most important gods, tricked them and made sure that only the gods drank the amrita and became immortal.
So, Kshira Sagar is basically a big, mythical ocean of milk that is the result of a massive, ancient event in Hindu mythology where the gods and demons worked together to get some really powerful treasures.