Someday refers to a future time when something will happen or be done. It's often used to talk about plans or hopes for the future.
When we use "someday" in conversation, it usually means that the thing we're talking about might not happen right away, but maybe someday soon. For example, if you tell someone you want to travel around the world, they might say "Someday I'll do that too!" It's a way of saying that you have big dreams or plans for your life, even if they don't seem possible right now. Sometimes "someday" is used in a more casual way to mean "maybe never", like when someone says "I'm going to run a marathon someday... maybe." But usually it implies that the thing will happen eventually.
Sometime; at some unspecified time in the future.
