Just live. Just love and be loved. I think that everyone understands this differently. For me, happiness is being in harmony with myself. I feel calm, I know what to do and I am in a good mood. This is happiness for me.
But the feeling of happiness is easy to lose if you do not listen to your feelings. It is important to feel happiness here and now. You don't need to think that you will be happy in the future. No, feel happy right now.
I know that sometimes we think that we have too many problems, or that there is no reason to be happy right now. But no. It doesn't work that way.

Happiness is right now and right here. Listen to yourself and understand that you are happy.