"Considering the multitude of data that people in our society need to remember,
a certain amount of notetaking and information deposited in books in unavoidable."
I know that 'a multitude of' means as same as 'a lot of',
but what does 'the multitude of' mean in this sentence?
I don't think the phrase "multitude of" is natural here. A multitude is a large number of individual items, be they people, birds, fish etc. Using multitude with data gives each individual bit of the data an emphasis that is uncalled for. Better would be the amount of data, or the quantity of data or for an emphatic statement the sheer quantity of data.