Simplify the following text for a Grade 13 native English-speaking person.
Original: Sikhs celebrate this festival because of the returning of their sixth Guru , Guru Hargobind Sahib Ji , from the Gwalior Fort , where he had been imprisoned along with 53 other Kings by Emperor Jahangir ( reason : being a non-muslim ) , and all of those 53 Kings had been freed along with the Guru on the Guru 's demand .
Simplified: For Sikhs , Diwali is important because it celebrates the release from prison of the sixth guru , Guru Hargobind , who also rescued 52 Hindu kings held captive by Mughal Emperor with him in the Gwalior Fort in 1619 .