What is ‘Emptiness’ (空）? Actually there are a variety of definitions in it, in some cases, there is also a totally opposite explanation. Yet it is a very essence of Buddhism. However the idea of ‘emptiness’ (sunyata in Sanskrit) is very hard to understand if you limited your cognision of the world just to this world. Instead of understanding this concept ‘emptiness’ intellectually, it is necessary to grab the idea with deep and spiritual insight.
To explain ‘emptiness’ simply, it is that we are essentially a spiritual being and life in this world is only temporary, then after we die we go back to the spirit world. So there is no fixed and firm existence in this world, everything is transient so that do not attach too much in this material world. Like a water mill we all go around this world and the other world. The bottom in the stream of river is this life, the top is in the other world. This is a concept of ‘emptiness’, it does not mean that there is nothing, but denotes that things transmute with the passage of time and therefore they have no solid or fixed substance.
As I mentioned above, there are other definitions that explaining in other ways, although the essence of buddhism is to have state of mind ‘nirvana’ by emancipating from this world without attachments (too much desires) in this world by knowing the truth of this life’s meanings. Unless you don’t understand this concept of ‘emptiness’ fully, if you just pursuit your happiness in the material world, you won’t have true happiness.