The Bible says that we know what love is, because we know who Jesus is, and what he did.
Love is a human condition. All humans need to give and receive love. It is one of the basic fuels that the human spirit was built to run on.
But there is a difference in the love of Jesus and his followers. That love somehow stands out. It is different from the love of "normal" human beings. It demonstrates the reality of Jesus being in a person's life and not being in a person's life. John says that we can recognize the reality of Jesus in a person's life by recognizing the reality of love in a person's life.