Those who have a why to live can bear almost any how.Friedrich Nietzsche — Twilight of the Idols, 1888
Meaning is the experience of one's life as mattering — not to some abstract standard or universal scale, but specifically: to people you love, to a project you care about, to values you have consciously chosen and consistently expressed. It is the difference between a life that is merely happening and a life that is being lived for a reason.
Viktor Frankl, who developed the most systematic account of meaning in the twentieth century, argued that the will to meaning is the primary human motivation — not pleasure, not power, not security, but the need to experience one's existence as oriented toward something worthwhile. His evidence was his own survival in Auschwitz, where he observed that the people who survived longest were not the physically strongest but those who retained a clear sense of something that required their continued existence: a person waiting for them, a work to complete, a testimony to bear.
He was careful to say that meaning cannot be invented or manufactured. It can only be discovered. The question is not what you want from life but what life is asking of you — what specific task, given your specific capacities and your specific circumstances, only you can do. The answer changes with circumstances. A person who has found meaning in their career may need to find new meaning after retirement. A person who has found meaning in raising children must find new meaning when the children are grown. The capacity to hear the new question, when the old question has been answered, is one of the most important and least discussed capacities in a human life.
The relationship between meaning and suffering is Frankl's most important contribution: suffering that is understood as meaningful — as the cost of something worthwhile, or as a testimony to a value that cannot be abandoned — is survivable in a way that meaningless suffering is not. This is not a counsel to seek suffering. It is an observation that the quality of how suffering is experienced is itself a matter of the attitudes one brings to it, and that those attitudes are, ultimately, a form of choice.