The summary is to give a general overview of what the property can do, whereas the value describes just that, what value to expect from the property.
Here is a good example of the difference on MSDN: List<T>.IList.IsFixedSize Property
Summary: Gets a value indicating whether the IList has a fixed size.
Value: true if the IList has a fixed size; otherwise, false. In the default implementation of List<T>, this property always returns false.
For the most part, summary tags will generally state “Gets or sets a value …”, whereas value tags will generally state what values to expect, including what the default value is expected to be.