When some task has been completed, usually by yourself. If someone is waiting for my attention and I tell them to wait a moment while I finish something, then afterwards I might say "Done! So what do you want?". Even if they are not waiting but they are aware of what I am doing, I might use "done" when it's completed. If something took little time or effort, I might also use "done" when I finish. For example, if someone was taking a long time to do something and I did it for them quickly, I might say "done" to say it's completed and emphasise how little time/effort it took. Similarly, if someone didn't want to do a task because it was a big deal for them (took too long or too much effort or maybe just lazy) then after completing it "done" again emphasises how quick/easy it was.