Genesis 48:20
And he blessed them that day, saying, In thee shall Israel bless, saying, God make thee as Ephraim and as Manasseh: and he set Ephraim before Manasseh. And he blessed them that day, saying, In thee shall Israel bless, saying, God make thee as Ephraim and as Manasseh: and he set Ephraim before Manasseh.
In the verse a father blesses his children by laying his hands reverently upon their heads. This was the act of a Jewish father when blessing his children. The laying on of hands means asking God to bestow His blessing upon those who receive it.
There is no power inherent in the act itself to impart the blessing. The laying on of hands by the ministers at Antioch upon the apostles meant the same thing. It was asking God to bestow His blessing upon them in their devotion to the specific work God had chosen them to do.