Yaakov had worked seven years for the hand of Rachel in marriage, but on the night, Rachel swapped with Leah so that Leah would marry Yaakov. Yaakov had suspected that Lavan would try and swap round Rachel with Leah, so had given Rachel certain signs (simanim) that she would tell over to Yaakov on their wedding night to make sure it was her. But Rachel gave these signs to Leah (Rashi 29;25) so her sister would not be embarassed.
But there is more to the story.
What were these simanim?
The da’as zekeinim says that they were the mitzvos of niddah, chalah, and candle lighting (the 3 ‘female’ mitzvos) – so Rachel taught them to her sister Leah. Later in the sedra, Leah accuses her sister of marrying Yaakov when she did not necessarily want Rachel as a co-wife (30;15). Leah did not know that Rachel had given her the simanim; she thought that Rachel just taught her hilchos niddah, chalah, and hadlakas haner for no apparent reason (she did not know that these were the simanim). And the greatness of Rachel is that not only did she do this massive favour to her sister without her sister even knowing what Rachel had done for her, but she never told Leah subsequently at all. She did not respond ‘you owe me one..’ or anything of the sorts.
That’s true selfless kindness.[heard from Rav Dovid Kaplan]