Dryden's Essay of Dramatic Poesy is a discussion in dialogue form of
such matters as the merits of Elizabethan, French, and Restoration
plays, the place of rhyme in drama, and the value of dramatic "rules."

It is said that the unity of time the audience can comprehend is
about 24 hours or as close to that as they can come. The reasoning
behind this is the fact the pretend action, or plot of the story should be
about the same as the time that it is representing, or as close as
possible. By doing this the viewer of the play is not getting what he or
her should be. By limiting the amount of time that can be represented
in a play, you are limiting the potential of the writer. And by doing this
you are not giving the viewer what they want, a good show.

The unity of place is where the play is set, where the action takes
place. This unity says that the play should have a consistent setting.
Since the stage on which the play is performed is only one stage, it is
inconceivable that it be thought of as many different ones, far apart or
near to each other. If the setting varied in position, being far apart,
would not hold true to the unity of time. If you were to travel to other
cities or countries it would surely take longer than twenty-four hours.
The French follow this strictly, a scene is never changed in the middle
of an act. If the act begins in a bedroom or on a street that is where it
is going to end.

The third unity touched upon is the unity of action. The unity of
action says that there is only room in one play for one major action.
That action is to be the aim of the play. Everything in the play has to
do with the completion of this action. If there are two actions in one
play it is not one play, it is then two. This does not mean there cannot
be many actions in the play. It means that all these actions must have
something to do with the overall, also know as sub-plots. If there is
one main action and that action is completed then the audience know
exactly what is going on. But if there are sub-plots and other things
that are not fully completed then there will be an element of suspense,
it will keep the audience interested.

If we were to compare literature of his time to these unities, I would
bet, very few of them would measure up. For example Shakespeare, look at
any one of his works. The unity of time is completely forgotten, some of his
works are stretched on for generations. Settings in one play go from one
country to another, city to city. And the actions in his plays can sometimes
be the actions of a mans entire life. So in short, John Dryden is saying that
the three unities can be, and are, forgotten.