This is a great question because we hear the term very often during severe weather season. Strong thunderstorms frequently have strong downdrafts associated with them.
A downdraft is basically a column of sinking air in a thunderstorm. When there is a particularly strong downdraft, we call it a downburst, or microburst. These microbursts contain significantly rain-cooled air and upon impact with the surface, the air moves outward in all directions. Think of it as a bomb falling from the air with the shock wave being wind. That shock wave is what we call straight line winds.
Another reason they are called straight line winds is because the damage created by them is usually very distinguishable. It is clear that there is no rotational damage pattern. For instance, a whole line of trees could be knocked over all in the same direction over a quarter mile as a result of straight line winds, whereas in a tornado, all the trees would be uprooted and deposited in various locations.
Straight line winds can blow up to 150 mph, and that's why it's important to stay away from windows during severe storms!