Last night I was reading to the kids and my girl asked me what the word "mine" meant. I explained to them that "mine" means "something that belongs to me". Then I gave them a few examples. With kids, I always explain using examples. It helps them to understand better.
I pointed to my pillow and said "This is my pillow" so "This pillow is mine." Then I pointed to the blanket and said "This blanket is mine". Then I asked them to give a few examples and the sweet talker (the boy) pointed to me and said "This mummy is mine." lol
The kids are getting more vocal now. Read about what they have to say when they think they are being given unfair treatment compared to their sibling.