Описание тега background-thread
A background thread is a thread that not directly driving the application but does some processing asynchronously.
Many UI toolkits require the use of background threads for anything that takes longer than a couple of milliseconds in order to keep the UI responsive.
Background threads are referred to as daemon threads at times.
Often background threads take care of recurring tasks, like cleaning up caches or similar.
Since the term 'background thread' is not very specific you might want to consider more specific tags related to the actual technology you are using.
If your question is independent of the actual technology this tag is a good fit.