In the context of the play, this line has several layers of meaning. On one level, it reflects the themes of revenge and violence that run throughout the play. Tamora's desire for revenge is fueled by her grief and anger over the loss of her son, and she is willing to go to any lengths to achieve it. On another level, the line can be seen as a reflection of the power dynamics at work in the play. As a woman and a foreigner, Tamora is often marginalized and underestimated by the male characters around her. However, her declaration of vengeance shows that she is not to be taken lightly, and that she is capable of wielding power and influence in her own right.
The line "vengeance is mine" also has a broader resonance beyond the context of the play. It has been used throughout history to express the idea that justice and retribution can be achieved through personal action, rather than through legal or institutional means. It is a call for action and a reminder that those who wrong others may not always escape the consequences.