[c#] What is a method group in C#?

The first result in your MSDN search said:

The method group identifies the one method to invoke or the set of overloaded methods from which to choose a specific method to invoke

my understanding is that basically because when you just write someInteger.ToString, it may refer to:

Int32.ToString(IFormatProvider) 

or it can refer to:

Int32.ToString()

so it is called a method group.