A great filter is anything that prevents nonliving matter from becoming living matter that undergoes evolution until it is smart enough and capable enough to colonize the entire galaxy, and possibly further. Presumably, life is subject to one or multiple of these filters, since otherwise we would expect the galaxy to be teeming with life.

People often make suboptimal decisions, based on their biases, laziness, or whims. These decisions then constrain their future actions, whether they intended that or not.