Two editors, one paragraph
Imagine two people edit the same sentence in separate copies of a document. When those copies are merged, a computer cannot tell which edit to keep. That is a merge conflict.
In code, this often happens when branches edit the same lines, or one deletes code that another edits.
A conflict is not necessarily a mistake
A conflict does not mean either person made a mistake. A developer reviews both changes, chooses or writes the right final version, and tests it. Small changes and short-lived branches can reduce conflicts, but not eliminate them.