The word "contains" refers to the act of holding or having something within. When we say that something "contains" another thing, it means that the first thing includes or is made up of the second thing. This can apply to physical objects, like a container holding liquids, or abstract concepts, like a book containing ideas or knowledge.
The word "contains" comes from the Latin "continere," where "con-" means "together" and "tenere" means "to hold." Over time, it evolved through Old French into Middle English, eventually becoming the modern English word we use today.