"Tuples in a weak entity set are partitioned according to their relationship with tuples in a strong entity" - meaning?
I believe what it's trying to say is because weak entity sets don't have their own primary keys, they are dependent on the strong entity sets that they have a foreign key relationship with (because those related strong entity sets do have primary keys).
So in other words, a tuple in a weak entity set will be partitioned (split up) from other tuples in that same weak entity set by their related strong entity set tuples.
I'm assuming the purpose of weak entity sets is a form of normalization by breaking related fields out of the strong entity set into a separate related set all together (a weak entity set without a primary key).