What gives human life meaning, and how should this understanding influence our actions?

Try taking a linguistic approach to this question about meaning. A word "tree" means the object or ideatreeonly because English speakers have agreed to call it "tree." But treeis expressed in other ways in other languages such as "baum" in German. So, there is no essential link between a word "tree" and the thing/idea tree. If the link between word and idea does not give something meaning, what does? 

From structural linguistics (initially Saussure), we learn that meaning derives from differences. "Tree" means treebecause of differences. You need other words to define (give meaning to) any word. To describe what a tree is, to someone who has no knowledge of trees, one would say or write things such as: a tree is a plant which makes its own food, a tree is rooted in the ground, some trees are deciduous, etc. As one continues to describe the tree, subsequent terms will have to be defined (i.e. deciduous) and more and more relationships will have to be explained. Theoretically, this could go on forever; even to the point when one is saying a tree is not a shark, a tree is not a flea, etc. This sounds silly, but to continue describing how a tree exists in the world, and what a tree is, one would have to say what a tree is not. 

The word/idea ("tree"/tree)means something because of its difference and relation to other words. Jacques Derrida describes this function of meaning as differance. Differance means to differ and to defer. Therefore, a word means something because it differs and defers to other words. This can go on forever as Derrida wrote, "All meaning is always already deferred." We have an individual word and to find its meaning we focus on that word itself and how it relates to other words. 

Analogously, the real object tree (in the world) has meaning/exists because of its difference and relation to other objects in the world. Consider a person as analogous to a word. A person is conscious and is therefore quite in touch with his/her views of the world, ethics, and so on. But in this analogy, a person might find more meaning by acknowledging other people and exploring those relationships. Just as a word derives meaning from its relationships to other words, a person derives/creates meaning by participating in and beyond his/her own social context. 

Saussure and Derrida never proposed to articulate a meaning of life. This is just one way to think about meaning itself and how it works. As for a meaning of life such as love, justice, or the human condition, that is a subjective project that a person should explore individually and with other people. Such is the analogy: a word gets meaning from relationships with other words, a person has meaning individually but that person's meaning in life will be enriched by acknowledging and participating in all manner of relationships: linguistic, social, intimate, political, spiritual, etc. 

What gives human life meaning, and how should one act upon realizing this meaning?

The meaning of life and acting upon life’s perceived purpose is a question pondered across many disciplines including philosophy, psychology, religion, art, science, and literature. All of these disciplines answer pertinent questions relative to life and its meaning quite differently. Yet, one unifying existential concept that many disciplines seem to share is contemplating the existence of a soul. One’s life meaning is determined and directed by the desires of the soul. Life’s course is plotted by navigating the conflicts between satisfying the soul’s desires vs. meeting the world’s demands. One short answer to questioning the meaning of life might be: It’s all relative. A few links below offer further insights.