It encapsulated the fact that a species could have high mortality at one point in its life cycle, then low mortality at another, while a complementary species might have low mortality at the first point and high mortality at the second. The more similar this term was for two species, the more likely it was that a pair could live alongside each other despite competing for space and nutrition.

TLDR: There are way more species than you might expect in a system with inter-species competition because different lifespans allow for niches to exist across time as well.

The actual paper