The Product Backlog is a list of product related tasks the team needs to work on within a defined period of time, as well as the tasks to be completed after the defined period of time.
It is important to prioritise the product backlog so it doesn't become an open-ended list of every random task or suggestions from the team.
Watch this video that explains the product backlog
An Example of a Product Backlog
Read up on how to prioritise a product backlog
Listen to this podcast on "the anti-pattern of long product backlogs"