The gemara (Meg 4) explains that although women are normally exempt from mitzvos that must be done only at specific times (zman gerama), they are obligated in the mitzvah of reading megillah because af hein hayu b’oso hanes. Rashi interprets the gemara to mean that women were also included in the miracle of being saved from the threat of Haman’s plans. Rashbam, however, interprets the gemara to mean that the concept of af hein to mean that women (or in this case a specific woman – Esther) were instrumental in bringing about the miracle, not just beneficiaries of its occurance. Tosfos attacks the Rashbam based on a textual nuance. The word af means even, secondarily. According to Rashbam, the gemara should not say even women were included in the miracle, as they were the primary agents in bringing about the miracle!
The Chasam Sofer (Toras Moshe, P’ Beshalach) suggests an answer to this question. One’s sense of what is miraculous is relative to one’s assumptions about what is to be expected in the natural course of events. Precisely because women had no doubts that G-d would bring about the redemption from Egypt, the escape of the Purim story, and the victory of Chanukah, they did not view these occurrences as miracles. Only those who doubted redemption were shocked at the remarkable turn events took and responded with hallel and praise to G-d. The gemara’s insight is that even women, who took for granted G-d’s help and initiated events that led to redemption, on some level af hein also felt something miraculous in the course of what occurred, and therefore are also obligated to celebrate.
Leaving this bit of pilpul and Tosfos’ linguistic nuance aside, what conceptually is the point of disagreement between Rashi and Rashbam?