The themes of this movie are two primary things:
- The new Replicants are what he original Replicants were meant to be - emotionless and obedient; and this movie explores the inevitability of emotional growth in the Replicants
- The "Child" - which acts as both a threat to the existing system, and a potential catalyst for change.
So firstly, the Child acts as a catalyst for emotional growth; depicted (primarily) through K. Replicants are built to be submissive, devoid of emotion to reduce conflict when given tasks to complete. Replicants are simply tools - nothing more.
At the beginning of the movie, K exhibits this kind of personality. Little care for anything that Sapper Morton has to say about a Replicant-born child; little more than curiosity. He's there to kill Sapper. That is his assigned task.
As the movie progresses, he begins to show signs of emotion. Happiness towards Joi, fear towards other officers as they scorn him and degrade him in the halls of the police station (though that could simply be a behavior to appear "inferior", to avoid confrontation in the workplace).
As the movie goes on, K starts exhibiting more emotional responses to situations - his curiosity grows, along with his fear as he begins to believe he is the Child, to the point where he explodes in anger and terror when he realizes the memory is the memory of the Child, and actively lies to save himself from the potential consequences of the situation.
This is also seen in a similar growth in Luv, Neander Wallace's "favorite". She starts off simply as a servant, cooperating as her role requires of her. But we quickly find that she has her own emotional motivations - fear and desire to find the Child, as it is Wallace's greatest desire to have it. Luv fears that if she is incapable of finding it, she will suffer severe consequences. This drives her fury as she is constantly thwarted in her search.
So, when K meets the Replicant Freedom Movement, Freysa is using this line to instill hope in K. This is the point where K changes his motive; from hiding through fear, into action through hope that he can do something about the whole situation.
Secondly, the Child is a symbol of freedom for the Replicants. Freysa was there when the Child was born. She, like Sapper saw what this meant - freedom from their roles as slaves. To be the Child would make you a savior, a living symbol, and an unstoppable force to lead the Movement, but most of all you would be real.
Being a Replicant is an emptiness. They are given the bare essentials to maintain their role. They have no childhood. They have no real memories of their own. To be the Child would make all of it real. To a Replicant, I can only imagine what that feeling must be like.