Khe Sanh is being attacked, and your mission is to push back the advance of the Vietcong. There are a lot of them, and a good chunk of this mission involves repelling dozens of enemies from overtaking your position.
Once you follow your squad leader to the first barricade, you have no idea where you're supposed to go or what you're supposed to do. Presumably you're supposed to make it to the bottom of the hill, where there's a waypoint marker, but running down there basically means instant death. But you try to do it anyway six or seven times because, well, that's what the game is telling you to do.
What you really should be doing is ignoring the waypoint and listening to your squad leader, who asks nearby soldiers if some nearby barrels have napalm in them. They say yes. But that's all you get. You try shooting the barrels to make them explode, but that does nothing. Grenades? Nope. Shotgun blast at close range? Nada. Meanwhile, as you're trying each of these things, you're dying again and again, starting at a checkpoint which requires you watch a 20 second introduction each time you die.
The secret to this mission is that you need to run up to one of the napalm barrels and hit the Use button, which causes you to slice the top open with your knife and kick it over.
Unfortunately the animation of slicing and kicking takes enough time that you're likely to be dead by the time it finishes. It seems the only way to do it successfully is to kick the barrels in a certain order. A secret order. One that you discover by...well...dying a lot more.