Double spending means spending the same money multiple times. In a physical currency, the double-spending problem can never arise. But in digital cash-like bitcoin, the double-spending problem can arise. Hence, in Bitcoin transactions, there is a possibility of being copied and rebroadcasted. It makes it possible that the same bitcoin could be spent twice by its owner. One of the primary aims of Blockchain technology is to eliminate this approach up to the possible extent.
Blockchain prevents the double-spending problem by implementing a confirmation mechanism from multiple parties before the actual transaction added to the ledger.