In a sense it's exactly what it says it is. In a non-plug in hybrid vehicle the only way to add charge to the onboard battery is via regenerative braking or by the gas powered onboard generator. So if you finish a drive in a non plug-in hybrid and you have 2 miles of EV driving range, or the state of charge is say 23% - that's what you'll have when you turn the car on the next morning.
The plug-in systems let you top off the battery while the vehicle is parked, and though it's not a rule of the sort, they usually also have a higher capacity battery for extended EV driving range.
Also - yes, a non plug-in hybrid vehicle can indeed propel itself only in EV mode, but the distances are usually pretty short.
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