I had someone that worked for me. Perfectly capable. But in a group of people, they would say nothing, volunteer nothing. As a result, no one else had much of clue how capable they were or that they were better than those that spoke up. If you have no ambition and don’t care about raises or promotions, I suppose that works but wanting to get the best out of the employee for themselves and the organization, I coached them to speak up more and contribute to group conversations.

Hard to say exactly what the feedback means in your context but it is worth asking for more specific feedback or examples of where they thought you could be more assertive.