When did you baby start speaking ? repeating words after you

My son is almost 15 months old and i don’t think he actually knows word . He can say ‘mama’ ‘dada’ but he doesn’t know what they mean he just babbles them sometimes. He’s really smart & meets all his milestones, but doesn’t ‘talk’ yet.

The only word I can think of that he might know is ‘No’ when I tell him no he automatically shakes his head no and stops what he’s doing but he doesn’t say the work back to me.

He babbles a lot. When I ask if he wants an orange (spoken or signed) he turns to where we have our fruit bowl. So he has a concept of what some words mean but he doesn’t speak them. Should I be concerned or is this normal ?