Let's read the passage first:
1 Peter 3:7 (NASB)
7 "You husbands in the same way, live with your wives in an understanding way, as with someone weaker, since she is a woman; and show her honor as a fellow heir of the grace of life, so that your prayers will not be hindered."
7 Likewise, ye husbands, dwell with them according to knowledge, giving honour unto the wife, as unto the weaker vessel, and as being heirs together of the grace of life; that your prayers be not hindered.
Christian husbands are to show their wives special honor as a fellow heir of the grace of life. They are to do this, in order to continue to have access to the Heavenly Father. When our relationship is not right with our spouse, our relationship with our Heavenly Father suffers.
The woman is called weaker, not because she is weaker in a moral sense, or a mental sense, or an intellectual sense, or an emotional sense. She is simply weaker physically.