In Java, widening primitive conversion is the process of automatically converting a smaller data type to a larger one. This is a type of implicit conversion, which means it occurs automatically without the need for explicit code.

Widening primitive conversion is often used when assigning a value of a smaller data type to a variable of a larger data type. For example, if we have an integer value of 5, we can assign it to a variable of type double by simply writing: 

int x = 5;
double y = x;

In this example, the integer value 5 is automatically converted to a double, which has a larger data range and precision.

The widening primitive conversion rules in Java are as follows:

• byte -> short -> int -> long -> float -> double
• char -> int -> long -> float -> double

It is important to note that while widening primitive conversion is generally safe, it can lead to precision loss in certain cases. For example, converting a float to a double may lead to a loss of precision due to the difference in the way the two data types store decimal values.

In summary, widening primitive conversion is a useful feature in Java that allows for the automatic conversion of smaller data types to larger ones. It is important to use it carefully to avoid unintended consequences.