A backlog is an ordered and prioritized list of everything that needs to be done within a project. A product backlog is a term used in scrum, an agile software development method.
The most important tasks are at the top and will be finished first. In simple words, a backlog is kind of like a to-do list.
The development team pulls tasks from the product backlog according to capacity. The product backlog is dynamically changed and adapted. When a time-critical task comes up, it will be placed on top of the product backlog and, therefore, it'll be processed first.
Sometimes people describe an overload of work or personal duties as backlog.
This gives the term a negative reputation. A healthy backlog is actually a good thing.
Orders or tasks can be accepted and do not have to be rejected. They come, so to speak, into the waiting queue. However, if deadlines are missed, the order backlog turns into overload. Here, backlogs are a bad thing. That's why it's important to meet these deadlines.