What are Yibbum ("levirate marriage" to a brother’s widow) and Chalitzah ("pulling off the shoe")?
According to Torah law, if a married man dies childless, one of his brothers must marry his widow. Any of the brothers can do this, but the oldest one has priority. This law applies even to brothers who are already married, because the Torah permits polygamy. If the brothers don’t want to marry the widow, any of them can dissolve the entire obligation by performing the ceremony of Chalitzah ("pulling off the shoe"), as described in Deut.25:5-10; the widow is then free to marry anyone.
Ordinarily, a man is forbidden to marry his brother’s widow (Lev. 18:16). The Torah permits it only when the brother died childless, and it must be done with the sole intention of fulfilling the Torah’s commandment. The Rabbis have ruled that people nowadays aren’t capable of having such pure intentions; for this reason, marriage to a brother’s widow (Yibbum) is no longer allowed, and Chalitzah is always performed instead.