In "The Things They Carried," why does O'Brien move back and forth between descriptions of what the soldiers are carrying and what Cross is thinking?
O'Brien's descriptions of what the soldiers carry in Vietnam are part of his characterization. What a soldier carries represents a unique part of his personality and also shows a part of who the soldier is in "The World"--life outside of the war.
His alternating between the descriptions and Cross's thoughts is a style he follows in several of his stories in this short story collection. See "In the Field" for another example. His purpose for using this style is to contrast the harsh reality and randomness of war and a soldier's life and Cross's desire to be back home where life is "normal." His thoughts are an escape from the heavy responsibility of leading his men and ensuring that they and the things they carry get home safely.
The way that O'Brien describes what the men carry have both literal and concrete meaning: in terms of what "things" the men carry are all of importance to the men, in so many ways. Some of those ways are both the same, but most are very different. Just because two men carry a poncho, that doesn't necessarily mean the weight of the poncho is the same. One man may carry it for rainy nights, and it may weigh 1 pound, but another man may carry the poncho so that it could be used as a tarp to put over a dying soldier from their subdivision. Because the man may think that's inevitable, or they fear it could be themselves, that poncho could have a "weight" of 30 pounds. This is a concrete object. Of all the things the men carry, they each have certain abstract "things" that they each carry; each one has a different weight. The things that these men carry, and the reason why O'Brien describes them in this way, and also commentates on the reasoning behind why they may carry that specific thing is because he wants to show how important that "thing" may be to each man. He creates a feeling or emotion that the reader can see throughout each character.