Overflow is what happens when content is larger than the space set aside for it. It is like packing too many books into a small box: some may stick out or need a different place.
A long word might stretch beyond a card, or a wide table might not fit on a phone. The page can let people scroll that area, wrap the content, or provide another way to view it.
Choose what happens deliberately. Hiding overflow can make important words or controls impossible to reach, so check both small and large screens.