Weak and Strong; Spontaneity

A reversible reaction is in fact built up of two reactions. Those two reaction are put in one equation with two arrows.
In general you may expect that on one side you can find the stronger substances and on the other side the weaker ones.

As the word suggests: the stronger substances have a stronger tendency to react than the weaker ones.
An now: substances that react will disappear, where the weaker ones are produced.
Isn't that very special? In chemical equilibria:

The stronger substances react in favor of the weaker ones

Please not that they do this spontaneous. Try to find a similar attitude in our modern society!

The weaker substances do react (back), but with little power, with not much yield or output.
Only with external power the weaker substances can be forces to react more completely.
When that happens, there is no spontaneous reaction.