You will often see permit holders only signs in busy cities such as London, where roadside parking is limited and only residents are permitted to park there. Local residents would be required to display a badge that shows they are permitted to park in the 'permit holders only' spaces.
Some permit holder only areas have time restrictions on them. For example, a sign might read:
"Resident permit holders only Mon - Sun 8am - 6pm"
In this particular example, you would only be allowed to park your car in this space between 8am and 6pm if you hold a valid permit. If you do not hold a permit you can park here any time before 8am and after 6pm.
However, these time restrictions can vary from place to place, some permit holder only signs only have 2-hour restrictions on them