What is “equal” in a chemical equilibrium?

Hint: Equilibrium is a state of balance or a stable condition in which opposing forces cancel each other out, and no modifications arise. There are two types of chemical equilibrium, which are Homogeneous Equilibrium and Heterogeneous Equilibrium.

Complete step-by-step answer:
Chemical equilibrium is the situation of a chemical reaction in which all reactants and products are present in quantities that have no further potential to change over time, resulting in no observable change in the system's properties. When the forward reaction and the reverse reaction each occur at the same time, this condition occurs. The rates of the forward reaction and backward reaction are usually not zero but equal. As a result, there are no net changes in the reactant and product concentrations. Dynamic equilibrium is the term for such a state.

We have the following equation for the generalized equilibrium:
$A + B \rightleftharpoons C + D$
The rate of forward reaction is ${k_f}[A][B]$ , and the rate of backward reaction is ${k_r}[C][D]$.
The state of chemical equilibrium is achieved when the rate of the forward reaction is same as the rate of backward reaction.

forward rate $ \equiv $ backward rate,
$ \Rightarrow \dfrac{{[C][D]}}{{[A][B]}} = \dfrac{{{k_f}}}{{{k_r}}} = {K_{eq}}$
${K_{eq}}$ represents the thermodynamic equilibrium constant.

The thermodynamic equilibrium constant, ${K_{eq}}$, must be calculated for a specific reaction at a specific temperature. The forward reaction rate and the backward reaction rate are "equal" in chemical equilibrium.

Note: Pure solids and liquids are not taken into consideration when measuring equilibrium constants. This is because of the fact that their concentration remains constant during the reaction. Both compounds are in the same phase in homogeneous equilibrium. Substances in heterogeneous equilibrium are in different phases.