What does negative delta G mean?

Hint: The amount of usable energy (energy that can do work) in a system is measured by its Gibbs free energy (G). During a reaction, the change in Gibbs free energy provides useful information about the reaction's energetics and spontaneity (whether it can happen without added energy).

Complete answer:
The change in the Gibb’s free energy can be written as:
$\Delta G = {G_{final}} - {G_{initial}}$
In other words, $\Delta G$ is the change in free energy of a system as it transitions from one state to the next, such as all reactants to all products. The maximum usable energy released (or absorbed) in transitioning from the initial to the final state is represented by this value. Furthermore, the sign (positive or negative) indicates whether a reaction will occur spontaneously, that is, without the application of additional energy.
 $\Delta G$ is calculated using a set of values that scientists can measure: a reaction's enthalpy and entropy changes, as well as the temperature at which the reaction occurs:
$\Delta G = \Delta H - T\Delta S$
Exergonic reactions are those that have a negative $\Delta G$ and release free energy. The reactants, or initial state, have more free energy than the products, or final state, with a negative $\Delta G$. Because they can occur without the addition of energy, exergonic reactions are also known as spontaneous reactions.

Note:
It is important to understand that the term "spontaneous" has a very specific meaning in this context: it means that a reaction will occur without the application of additional energy, but it doesn't specify how quickly the reaction will occur. It could take seconds for a spontaneous reaction to occur, but it could also take days, years, or even longer.