What is Type Casting

Conversion between different data types can be done explicitly using a cast. A cast explicitly invokes the conversion operator from one type to another. The cast will fail if no such conversion operator is defined.

Have a read here:
MSDN: Casting and Type Conversions (C# Programming Guide)[^]
WikiPedia: Type conversion[^]

Type casting is changing a variable from one type to another, e.g. from float to int.

You'd use it when you have an object of one type but you need it to be another, but it is often handled for you, for example if you used

String.Format("{0} {1} {2}", 1, 2.3, "four");

String.Format is expecting a string and 3 objects, but it was given a string, an int, a float, and another string instead, so the last 3 arguments are cast to objects instead.

Another time would be if an object explicitly implements an interface, it must be cast to that interface before the interface's methods can be used.

Like in this case:

class IntComparer : IComparer<int>
{
    int IComparer<int>.Compare(int x, int y)
    {
        return x.CompareTo(y);
    }
}

If you made an object of type IntComparer, the Compare method is only available if you cast it to an IComparer<int> first, like this:

IntComparer c = new IntComparer();
c.Compare(1,2); // not valid!
((IComparer<int>)c).Compare(1,2); // valid