In the context of saying, “you’re arguing semantics,” it means that your argument is due to you disagreeing on the meaning of the words used rather than your actual positions.
For example, arguing over whether a tree chopped in the woods when no one is around to hear it still makes a sound is a semantic argument. Everyone agrees that there is sound as “vibration of air” but no sound meaning “perception of the vibration of air.” The argument is entirely about what you believe the symbol “sound” means.