what does the title mean in the things they carried?explain the meaning of the title
The first part of the answer is the easiest: O'Brien's title, in part, refers to all the items carried by the soldiers in the stories. For example, they all carry items necessary to do their jobs, such as their rifles, knives, can openers, water purification tablets (and Kool-Aid to kill the taste of the purification tablets). As individual soldiers are introduced into the stories, we see that some carry bibles (Kiowa, for example), and Ted Lavender even carries tranquilizers to help him deal with fear. The medic, Rat Kiley, along with all of his medical equipment, carries M&Ms to give to soldiers who are fatally wounded and may find some hope in the "medication"--in Vietnam, many medics really did carry M&Ms for those situations.
The second set of things they carried is much more complicated. Many of the soldiers carry talismans to keep them safe: Lt. Cross carries a pebble he received from Martha, which he sometimes puts in his mouth; the machine gunner wears his girlfriend's stocking around his neck. Carrying such talismans is a typical way a soldier tries to enhance his survival and control his fear--he's hoping that the pebble or the stocking will somehow protect him.
Among other things they carried, O'Brien tells they "shared the weight of memory. . . . Often, they carried each other, the wounded or weak," that is, as a unit they've become like one person, and as a single entity, they carry all the memories of what each individual soldier has done or has suffered. When one soldier is wounded or killed, they are all diminshed, and they carry this knowledge with them always. They even carry the fear of being afraid and not doing their jobs, which is arguably the most pervasive fear among combat troops.
The things they carried, according to O'Brien, were not only physical things like weapons, talismans, medical gear, and water but also, and more important, such things as fear, love of each other, love of life, and a drive to keep in other alive.
The title has several meanings. The literal meaning refers to all the objects, both necessary and unnecessary, that the American troops in Vietnam carry around with them. Some of these objects are practical, such as binoculars and dog tags, while others are objects that bring to comfort themselves and confer luck as charms, such as letters from a girl back home or a girlfriend's pantyhose around their neck.
What they carry also has symbolic meaning. Toting around their objects symbolizes their attempt to bring home to Vietnam with them. For example, Lieutenant Jimmy Cross brings letters from a girl named Martha back home, and he also carries around a pebble she sends him. It is as if the soldiers are trying to bring these elements of home into Vietnam to ensure that these objects will give them some of the safety of home.
What the soldiers carry also indicates how much baggage, physical and emotional, they carry with them. Much of what they carry are weapons of war, including a "28-pound mine detector" and "one-pound blocks of pentrite high explosives, four blocks to a man, 68 pounds in all." These objects are weighty and symbolize the soldiers' apprehension and inability to fend off the enemy except with brute force that may or may not be effective. By carrying these American weapons of war, the soldiers also carry the burdens of the conflict in Vietnam, in which the soldiers attempt to destroy the enemy without really even knowing where the enemy is.