Part of one plant is joined into the stem, branch or rootstock of a different plant, in such a way that a union is formed and both plants grow together! Fruit trees and roses are commonly propagated this way, and I expect there are numerous other examples.
Getting back to your roots:
The Gentiles developed an arrogant supposition that God had abandoned His Jewish nation Israel, in favour of a new Gentile-only covenant. We recognise this as early anti-Semitism against the Jewish people, which has infiltrated Christianity for thousands of years.
But in his letter to the Romans, the Apostle Paul wrote about grafting. Essentially, Paul was saying that if the rootstock (referring to Israel) is holy, so are the branches, but when a wild branch (referring to the Gentiles) is grafted in, then both branches (the holy and the wild) become equal sharers in the rich root of the olive tree. His analogy using the olive tree wasn't lost on the Jews - for they knew their Hebrew heritage was symbolised as God's Olive Tree. "The Lord called your name Green Olive Tree, lovely and of good fruit." (Jeremiah 11:16) "With the noise of a great tumult He has kindled fire on it, and its branches are broken."
Paul went further with his horticultural example:
He warned the Jews they were in danger of being 'broken off branches' if they stopped trusting in their Messiah."Don't boast as if you are better than the Jewish branches! Remember that you are not supporting the root - the root is supporting you!" (Romans 11:18)