The nouns ‘material’ and ‘materials’ are both the plural of ‘material’.
We use ‘material’ when we refer to abstract, uncountable ideas, like a group of ideas, ‘all of the writing material.’

On the other hand, we use ‘materials’ to refer to countable, concrete nouns like ‘building materials.’

This Contract contains classified material, so you must handle it with confidentiality.
The construction workers need better building materials for the renovation.