Wikipedia explains Grafting as: 

"…  a horticultural technique whereby tissues of plants are joined so as to continue their growth together. The upper part of the combined plant is called the scion (/ˈsaɪən/) while the lower part is called the rootstock. The success of this joining requires that the vascular tissue grow together and such joining is called inosculation. The technique is most commonly used in asexual propagation of commercially grown plants for the horticultural and agricultural trades."

Fruit Tree Grafting For Beginners Youtube video by: Alberta Urban Garden Simple Organic and Sustainable