Each column represents the total military spend of the world's countries in that year. Each column is further broken into what separate parts spent that year, the categories for those parts being America, Russia/USSR, China, NATO except America, and the rest of the world.

So to pick some examples out, we can see Soviet expenditure (the bright red section of each column) crash enormously when the country fell apart in 1991. Post-Soviet Russia hardly spent anything by comparison, so the red section of each column got really small all of a sudden. This combined with the reduction in American expenditure (the pale blue section) in the 90s resulted in a low total spend for the world (the total height of each stack), even though the spending of every other country outside of those two stayed fairly constant (the pink for China, dark blue for non-USA NATO, and grey for everyone else).