A generelisation is an overarching category between multiple (specialization) entities. All the specialisation entities also have the attributes in the generalisation entity. For example
A generalisation could be Vehicle. With an attribute ‘No. of wheels’
Two specialisations could be ‘Car’ and ‘Bicycle’. Both are vehicles and can use the ‘No of wheels’ attribute. You can also add attributes specifically for these specializations now, like ‘engine type’ for cars and ‘Frame type’ for bicycles.
Hope this makes it more clear.
generalisation is like “heritage”.
you can also understand it that way with an example :
account is a generalisation of user
this mean that an account is a user, but a user is not an account.
therefore when you create a user you are not creating an account. but when you are creating an account you are creating a user.
Also, because account is a user, account will have the same attributes as user + his own attributes
I hope this helps