Both the moon and the sun exert some gravitational forces on the Earth... and us on them!
During a new moon, the sun and moon are directly pulling in the same direction, exactly lined up:
S——m-—E
During a full moon, they pull in exact opposite directions:
S————E-—m
So there is some theory that the directional conjuction of the pull of the sun and the moon combine particularly strong during the full and new moons.
But the statistical analysis doesn’t really support that theory. Only marginally.
Good instruction!