A bullet graph compares an actual value with a target in one compact bar. The bar is the actual. A short black line marks the target, and gray bands behind the bar show how close you are, by default at 60% and 80% of the target.
Show Me builds it from two measures: select the actual and the target, then pick bullet graphs. Tableau puts the first measure on the axis as bars and the second on Detail, then adds a reference line per cell at the target and a distribution of 60% and 80% of it.
Which measure becomes the bar depends on the order Tableau reads them, and it's easy to get them swapped: the target as bars and the actual as the line. Right-click the axis and choose Swap Reference Line Fields to fix it.
You can also build one by hand: the target on Detail, then Add Reference Line with the scope set to Per Cell and the value set to the target.