Shortly, inheritence refers to extending/changing the functionality of a class without reimplementing the class. That means, if you have class A and you want to add more (or to change) functionality, you define class B: (inheritance access level: public, protected or private) A which is read as "B inherits A". Depending on the inheritance access level you have or not access to some members of class A (called base or parent class) in class B (called derived or child class).
When you have a cascade of inheritance, it is called multilevel inheritance (e.g., C inherits B which inherits A).
When you inherit more than one class, it is called multiple inheritance (take care of the diamond problem).
When you combine a multilevel inheritance with single inheritance, it is called hybrid inheritance (e.g., D inherits B and C, with B inheriting A).