An Emulsifier which is one of the important property can be explained in many ways. Emulsifiers are used when one part is water and another part is oil based, and you want to mix both this parts and would like to form an emulsion. It is one kind of substance which stabilizes an emulsion by increasing its kinetic energy for example (surface acting agents - Surfactants).
Suppose if you keep oil and water is a same plate or a glass, but after some time you will find that they would not mixed properly, you will find separate phase , one phase with water and other phase with oil, only little part of both of them will mix at the middle, which is known as surface tension. One barrier or a fence will create between both of them , Now we want to overcome with this problem , so what is the solution? The answer is to use emulsifiers which will allow to mix both this immiscible phase.