yes I know what they are. Use them all the time. But since the post is about if I imagine it's about things like

  if( x < 0 || x > 5 )
    throw new ArgumentOutOfRange();
  if( y == nullptr )
    thwrow new NullRefException();

which should imo be something like

  Contract.AssertInRange( x, 0, 5 );
  Contract.AssertNotNull( y );