The [ ] represent the concentration in moles/L
If the molecules in the reaction have coefficients, these will become exponents for the molecules in the Keq formula. For example, if 2A + B <----> C + D the formula would look like this : Keq = [C][D] / [A]²[B]. Remember, this formula always calls for products OVER reactants.
Now, lets look at an example:
2NO2 <--> N2O4
[N2O4] at equilibrium = 0.00140 M
[NO2] at equilibrium = 0.0172 M
Keq=[N2O4] / [NO2]²
Keq=(0.00140)/(0.0172)²
Keq=4.73