glasses terminology??????

I am living in saudi arabia. so here the term use for glasses power is -1 or -2 etc . what does this 20/20 means.
Suppose if i can see very poorer without glasses but with the help glasses or contact lenses i can see clearly so it is ok in aviation?????????
Suppose if i can see very poorer without glasses but with the help glasses or contact lenses i can see clearly so it is ok in aviation?????????
