The lyrics "I'm not making the same mistake / Won't be putting my heart on display" refer to past relationship trauma that has made him hesitant to be vulnerable.
"Charlie Be Quiet!" is a vulnerable pop-rock track by Charlie Puth that explores the of wanting to express deep feelings for someone while being terrified that doing so will drive them away. Meaning and Context
Puth uses "word painting" to match the music's volume to the lyrics' emotional intensity.
He worries that if his partner realizes how much he loves her, she will "run, run away," leading him to "water his feelings down" to stay safe.