What does it mean that the wife is the weaker partner? We’re not sure because the text doesn’t specify, but we know it isn’t saying that women are weaker morally, spiritually, or intellectually. The Bible teaches that men and women are equal before God. Later in the same verse it says that women are “coheirs of the grace of life.” Galatians 3:28 says, “There is no Jew or Greek, slave or free, male and female; since you are all one in Christ Jesus.”
Some scholars believe it is just pointing to the reality that in general, women are physically weaker than men, and, so this verse is encouraging husbands to act as their protectors. That's a good message for husbands, but I don't think it is the point of this text.
I think Peter is referring to their position of weakness in marriage. In a company, the employees are in a position of weakness, and the managers and owners are in a position of power. In a family, the children are in a position of weakness, and the parents are in a position of power. In society, civilians are in a position of weakness, and police, politicians, and judges are in a position of authority. And in marriage, the wife is in a position of weakness, and the husband is in a position of power. In marriage, the wife’s role is to submit and support; she is to follow her husband’s lead; she is to submit to her husband in everything, as unto the Lord. So in terms of the marriage, the husband is in a position power and authority, and the wife is in a position of weakness, vulnerability, and dependence. Her overall wellbeing is highly dependent on the husband.
What Peter is saying is this: In light of the fact that your wife is the weaker partner, and in light of the fact that she is highly dependent on you for her overall well-being, be a great husband.