In object-oriented programming, a class is said to inherit from another class when it is based on the implementation of that class. In Roblox's case, sub-classes inherits all the members (properties, methods, events and callbacks) of the superclass.
All classes in the Roblox object system inherit, directly or indirectly, from the Instance class. Many classes inherit from structural classes that provide members common to all their subclasses. For example, 
Part, 
WedgePart, 
TrussPart and 
VehicleSeat all inherit their classes from BasePart.