What is the Backlog?
The backlog is the entire list of tasks. You should add all the tasks you know of. As soon as you have more details, add them. If you receive an email from someone or make a decision at a meeting, enter the information in the backlog. The backlog should be sacred. You should check it regularly, keep it up to date, and make sure everything ends up there.
Prioritize your tasks
The order also plays a role. Move important tasks up. In Scrum projects your team will always take the top tasks into the next sprint. If you want something to be done next, move it up.
What is the benefit?
For your team, the backlog becomes the single source of truth. You will notice that it sorts your everyday life and you have your head free again for other things.