The climax of a poem, as in the climax of a short story or a novel, is the high point, or the most important point, that the poem is trying to communicate. It is therefore often also the most emotionally-moving or dramatic moment of the poem. In essence, the entire poem was working toward this moment, the climax. For example, in the old classic poem "Casey at the Bat" (check it out!), the poem builds suspense until the great heroic superstar baseball hitter, Casey, goes to the plate and swings at the ball. I won't give away the ending of the poem, but the ending is the climax. Everything builit up to it. You'll see, when you read this poem, that a climax sometimes comes as a surprise to the reader.