What is the meaning of "All the world's a stage"?
One of the most famous speeches in all of Shakespeare, "All the world's a stage" (also called "The Seven Ages of Man" outside of the play) is about the cycle of an average life. Spoken by the cynical Jaques in act 2, scene 7, it paints a bleak view of human beings.
The notion that the world is a stage evokes a sense of falseness and inevitability when it comes to what happens in a person's life. A play is already laid out before the actors take on their roles, and Jaques claims this is so for human beings as well. He argues that everyone (or at least, every man) goes through the same stages in their lives: the infant, the schoolboy, the lover, the soldier, the justice, the pantaloon, and, lastly, senile dotage and "oblivion." This presents the sense that every life is predictable and even foolish, since Jaques does not see fit to give any of these seven ages any positive attributes.
Cynicism aside, the speech also emphasizes the role-play predominant in As You Like It. After all, Rosalind is playing multiple parts during her time as Ganymede. Additionally, her role-play does not end in Jacques's empty futility, but in both the restoration of her previous fortunes and a marriage with the man she loves.