For a start, have you studied this topic yet? Have you understood that many reactions will go both "forward" and "backward" rather than reactants to products only?
A dynamic equilibrium is a state which a chemical system achieves, when the rates of both the forward and backward reactions of it are equal, at constant conditions.
This means that when this state is reached, there's no overall change in the concentrations of either the reactants or the products as the rates are equal - hence, both reactions are in equilibrium.
An equilibrium in itself is a phenomenon that, in a broad sense, gives you an idea of which side of a chemical reaction will be favoured due to different conditions - if the equilibrium of the reaction A + B ⇌ AB lies towards the right of the equation, then AB will be favoured, and vice versa for if it lies towards the left.
This comes back to the concept of a dynamic equilibrium - it's 'dynamic' as the equilibium is liable to shift based on different conditions, hence why I mentioned that equilibria are only reached at constant conditions - if you reached equilibrium and changed some conditions, then the equilibrium would shift to restore something known as the equilibrium constant.
But as mentioned, this is a broad question, and I think it's more to do with the fact that a good few equilibrium constants exist for different equilibria and different systems, and that there are a few ways that they're shifted and restored based on which conditions are changed.