I have a simple question. what are the meaning of :

half4 myHalf;
myHalf.yyyy;
myHalf.xxzz;
// or
myHalf.yw


Thanks a lot !

It's a simple answer! half4 is a four component vector, using half precision floating point coordinates. The extensions you've shown are quick ways to create new vectors using the values from myHalf.
myHalf.yyyy is a four component vector with the values: (myhalf.y, myhalf.y, myhalf.y, myhalf.y)
myHalf.xxzz is a four component vector with the values: (myhalf.x, myhalf.x, myhalf.z, myhalf.z)
myHalf.yw is a two component vector with the values: (myhalf.y, myhalf.w).