An out-of-place artifact is something that people find that doesn't seem like it should be where it was found. It's like finding a toy in the middle of a forest. It's unexpected and unusual.
Archaeologists and historians discover many things that tell us about how people lived in the past. But sometimes, they will find objects that don't make sense. For example, they might find a phone in a tomb from ancient Egypt. That's weird because there were no phones back then. It's like finding something from the future in the past.
Sometimes, people say that these out-of-place artifacts can't be explained by science and must have been left by aliens or some other mysterious beings. But scientists say that these artifacts are just things that people from the past might have made but we haven't discovered yet.
In conclusion, an out-of-place artifact is something that doesn't belong where it was found, and it makes scientists ask questions about the past.