The Product backlog is a set of all baseline requirements prioritized in order which is made available by the Product Owner to the Scrum Team. The product backlog emerges and evolves over time and the Product Owner is responsible for its content and validity.
Scrum teams work on the Product backlog and create a potentially shippable product increment.
Product Backlog maintenance, also popularly known as Backlog grooming, is an informal Scrum team ceremony that happens every sprint, in which everyone in the team, including Scrum Master and Product Owner get together to ensure that work items in the backlog are relevant and useful, and each items aligns to the overall product roadmap. It is indeed more of a working session than a typical closed door meeting.