Baptisms for the dead, or proxy baptisms, are baptisms performed in the temple for those who died before they had a chance to be baptized here on earth. You can perform baptisms for your own ancestors if you prepare and bring their names to the temple. Or temple workers can provide you with pre-submitted names.
A proxy baptism is similar to a normal baptism: you will enter the temple baptismal font, where a temple worker with priesthood authority will state the words of the ordinance and immerse you in the water. However, in addition to saying your name, he will also say the name of the person for whom you are being baptized.
If time allows, you may perform multiple baptisms during your baptismal session—think of all the people you can help! Keep in mind, however, that your decision to perform the ordinance does not force anyone to accept Jesus Christ and His Church. People in the spirit world can choose to accept the baptism or not. The good thing is they now have the opportunity to do so.