This phrase means you can't change someone's habits or teach them new things if they're old. Imagine your grandma who loves to watch TV all day. She's not going to start playing video games just because you want her to.
It's like trying to teach an old dog new tricks. The dog is used to doing one thing, and it's hard for it to learn something new.
This phrase is often used to say that some people are too old or too set in their ways to change or learn new things.
