As his fingers brushed against her neck, a mark was placed on her neck, appearing similar to a collar.
While the Servitude Mark was being created, Ghost Queen could easily break it by just resisting. However, she also understood the consequences of resisting at this movement.
Even though her heart was telling her to resist and not place her life in another person's hand, her mind was telling her that this was the right choice. She did not resist the mark, listening to her mind in the end.
After a few minutes, the Servitude Mark was completed and the Ghost Queen became a servant of Eren. With the Mark around her neck, she couldn't do anything that might bring harm to her master.
Although the mark restricted her freedom to an extent, it didn't enslave her. She still had the right to reject his commands if they were something that might lead her to death.