The property of equality that justifies the step in solving the equation
\[ 3z^2 - 5 = 10 \ \]
to
\[ 3z^2 - 5 + 5 = 10 + 5 \]
is the Addition Property of Equality. This property states that if you add the same value to both sides of an equation, the two sides remain equal. In this case, 5 was added to both sides.