Here is what we wrote in the Background notes to Kidushin 45:6

6) [line 12] CHALITZAH

(a) If a married man dies childless and has brothers who survive him, his widow or widows may not remarry until one of her husband's brothers performs Yibum (levirate marriage) or Chalitzah (levirate release) with one of them, as it states in Devarim 25:5-10.

(b) If the live brother does not wish to marry the dead brother's widow, he must perform Chalitzah. To do so, he appears before a Beis Din of three and states, "I do not want to marry her," after which his sister-in-law approaches him before the elders, takes off his right sandal and spits in front of him. She then declares, "This is what shall be done to the man who will not build up a family for his brother." After this process is completed, she is free to marry whom she wants (although she is prohibited mid'Rabanan to marry a Kohen). A woman who has been through such a process is known as a "Chalutzah."

M. Kornfeld