In plants there are special types of tissues called meristematic tissue.
The meristematic tissue are three types which are -
1) Apical meristem (responsible to increase the height of the plant called Primary growth).
2) Intercalary meristem (responsible for increasing the hight of the internode i.e. also shows primary growth).
3) Lateral meristem (responsible for increasing width of the stem i.e. secondary growth).
Cambium is one of the types of lateral meristem. Cambium itself is of two types 1) vsacular cambium (present between the vessels i.e. xtlum and pholem) and 2) cork cambium (present below the epidermis).
In grafting cambium helps in binding the two parts of the stem. Monocots do not posseses cambium hence grafting is not possible in monocots.
Grafting is possible in dicots because it posseses cambium.