The Baily's beads effect is a feature of total and annular solar eclipses.  The effect is visible for a few seconds just before totality begins and just as totality is ending.  The Lunar topography is very rugged and the limb, or edge, of the Moon's disk is ragged with high mountains and deep valleys.  This unevenness is the root cause of the Baily's beads effect.  As the Sun's photosphere is almost completely "masked" by the Moon it is visible through the "valleys" and other depressions along the edge of the lunar disk.  The bright surface of the Sun appears as beads, or dots, making a fantastic and rarely seen vista.  When only one bead is left, another awe-inspiring view develops:  The Diamond Ring effect.
The phenomenon was first explained by Francis Baily in 1836.

¹https://en.wikipedia.org/wiki/Baily%27s_beads