What if you can't see the 20/50 line in a snellen chart? does it mean you have poor vision?

Possibly: If this is you uncorrected vision, then you might just need eyeglasses - this is not poor vision in this case. If 20/50 is the best possible with the best correction, then there is a problem which your ophthalmologist needs to address. It is still not "poor vision" but is certainly less than normal.

Poorer than 20/40.: Means that you see at twenty feet what a well sighted person might see at fifty feet. It means that your vision is less than someone with 20/40 vision, but better than someone with 20/60 vision.