A more thorough explanation:

Manumit

To release a person from slavery.

• After years of being enslaved, the kind-hearted master decided to manumit his slaves and give them their freedom.
• The abolitionist movement fought for the government to manumit all slaves in the United States.

The examples show how manumit means to free someone from slavery. In the first example, the master decides to manumit his slaves, which means he is releasing them from their bondage. In the second example, the abolitionist movement is fighting for the government to manumit all slaves, which means they want the government to free all slaves in the country.