After Cain fears being killed, Genesis says the LORD put a mark (or sign) on Cain so that no one who found him would attack him Genesis 4:14–15. The text presents it as protection alongside Cain’s judgment of wandering.
In Genesis 4:13–16, Cain responds to Yahweh’s announced consequences with a desperate complaint: the burden is more than he can carry. He pictures himself expelled from the land, cut off from Yahweh’s “face,” and forced into unstable roaming, where anyone who finds him might kill him. This scene follows the first murder (Cain killing Abel) and Yahweh’s direct confrontation with Cain, including a spoken consequence that life will be difficult and unsettled (earlier in the chapter).
Cain does not deny what happened; he reacts to the announced outcome as unbearable (verse 13).
A careful answer keeps the immediate passage, the book's argument, and the whole-Bible storyline together, so the question is answered in context instead of as a detached idea.
Start with Genesis 4:14-15, then read the surrounding section so the answer stays connected to the passage's flow.
14Behold, you have driven me out this day from the surface of the ground. I will be hidden from your face, and I will be a fugitive and a wanderer in the earth. It will happen that whoever finds me will kill me."
15Yahweh said to him, "Therefore whoever slays Cain, vengeance will be taken on him sevenfold." Yahweh appointed a sign for Cain, lest any finding him should strike him.