What does 1 Samuel 17:39 mean?

David strapped on Saul’s sword over the armor and tried walking around, but he wasn’t used to it. He said to Saul, 'I can’t walk in these. I’m not used to them.' So he took them off. 1 Samuel 17:39 - Modern Text Bible

(David realizes he can’t fight in someone else’s armor; he needs to be himself.)