For a given equilibrium reaction, the equilibrium constant tells you what the ratio between the equilibrium concentrations of the products and the equilibrium concentrations of the reactants, all raised to the power of their respective stoichiometric coefficients.
For a general equilibrium reaction
#a"A" + b"B" rightleftharpoons c"C" + d"D"#
the equilibrium constant takes the form
#color(blue)(K_c = ( ["C"]^c * ["D"]^d)/(["A"]^a * ["B"]^b))#
The important thing to keep in mind here is that all those concentrations are equilibrium concentrations.
Now, the reaction quotient,
You can think of the reaction quotient as being a snapshot of how the equilibrium reaction proceeds at a given moment in time, let's say
#color(blue)(Q_c = ( ["C"]_t^c * ["D"]_t^d)/(["A"]_t^a * ["B"]_t^b))#
The concentrations used here are specific to the products and reactants at a moment
This means that in order for equilibrium to be established, the reverse reaction must be favored. Simply put, the equilibrium will shift to the left, favoring the conversion of products into reactants.
So, for a given equilibrium reaction, the reaction quotient will tell in which direction that equilibrium will proceed from the moment you calculated