With plants, it means to grow a different plant on a root. By making some cuts of the right kind you can graft an orange branch onto a lemon tree, so that one part of the tree produces lemons and another part produces oranges.
if you are talking about maths, it basically means the average. To find the mean of a group of numbers, you have to add them all together and divide them by the number of numbers there are. I know...
They don't "mean" anything, other than the replacement of the normal words that appeared in the original line. Are you thinking of this... "Come to the dark side" it's star wars language, look it up....