What does the main-sequence turnoff point on a CMD reveal about a cluster?

The mass of stars just running out of core hydrogen, yielding the cluster's precise age.

In the context of dating a star cluster using a color-magnitude diagram (CMD), the main-sequence turnoff point is a critical diagnostic feature. This point marks the precise location on the diagram where the most massive stars within that cluster—those that formed at the cluster's inception—have finally exhausted the hydrogen fuel in their cores and are beginning to evolve away from the main sequence toward becoming giants or subgiants. Because the relationship between a star's mass and its main-sequence lifetime is extremely well-established by physics, knowing the luminosity and color (hence, the mass) of the stars at this specific turnoff point allows astronomers to calculate, with high precision, the total time elapsed since the entire cluster formed simultaneously. This allows for an age determination accurate to within five to ten percent for the entire population.