It marks the exact place where Guru Tegh Bahadur, the ninth Sikh Guru, was publicly beheaded on Aurangzeb's orders in 1675. (A gurudwara, in Sikhism, is a place of worship.)
What happened there, in the chapter's own account. In 1675 a group of Kashmiri Pandits approached Guru Tegh Bahadur seeking protection from religious persecution. He decided to stand with them and court martyrdom. He was arrested, and Aurangzeb ordered him to convert to Islam. He was tortured, and made to witness three of his disciples being tortured to death. He refused. On Aurangzeb's orders he was publicly beheaded in Chandni Chowk.
What followed. His son Guru Gobind Singh — the tenth and last Guru — established the Khalsa in 1699, “a martial brotherhood committed to justice, equality and defence of the faith”.
How it is remembered today. The gurudwara is “uniquely honoured by the Sikh Regiment, which has saluted it before the President in the Republic Day parade each year since 1979.” The chapter calls it “a powerful symbol of faith and sacrifice in Indian history”.