After Jesus died, did he descend into Hades to set the captives free? If so, who were the captives?
There are two answers to your question, one general and one specific. In Luke 4:18 Jesus said that His mission was to proclaim freedom for the prisoners and to release the oppressed. This is a general reference to freeing all believers from the bondage of sin and the impossible requirements of religious work by paying for our sins on the cross.
Specifically, Matt. 27:52-53 says that after the Resurrection many holy people who had died came out of their tombs, and Ephesians 4:8 tells us that “When He ascended on High He led captives in His train.”
Before the cross, the spirits of all dead people went to Hades, the abode of the dead. After He had paid the price for our sins with his death, Jesus went there too (Luke 23:43). There were two sections there, one called Paradise where believers went and one called Torments where unbelievers still go. Those who had died in faith that He would one day come to redeem them were released from their temporary place in Paradise and when Jesus ascended into Heaven, they went with Him.