These are the four noble truths of Buddhism. Burn them into your mind.
1. There is Dukkha (unsatisfactoriness)
2. Dukkha has a cause
3. Therefore, Dukkha has an end
4. There is a path to the end of Dukkha
Dukkha is the inherent unsatisfactoriness in life. That no matter what we have, it isn't enough.
The cause of dukkha is tanha (this literally translates to thirst but is more accurately desire, craving or longing). Tanha is that no matter what we attain or achieve we always want more. Our pleasures and successes are fleeting.
The end or cessation of dukkha is Nibbana (more commonly known as the Sanskrit, Nirvana). The extinguishing of the flames of desire. When X ceases, so does Y. When tanha fades, so does dukkha. Nibbana is not something you can attain. It is something you already have. Your reality as is, is already Nibbana. The opposite of Nibbana, Samsara, is an unreality. It is a construct that you fabricate. The notion that this present moment is not enough. When the veils of thought, emotion and mental effort subside the peace that is your true nature emerges like how the sky does when the clouds part. Ironically, the seeking, the striving towards happiness, is what caused you to overlook the peace that was already there.
The path to the end of dukkha, is the Dhamma, the Buddha's teachings. These involve training yourself to pay closer attention to your present experience. What is it like to be you in this instant? What about now? This continual bringing of the attention away from one's desires of past and future (the unreality) and toward the present (the reality) is what we need to train ourselves to do. This can be called meditation.