What does double spend mean?
The spending of a single token (a Bitcoin for example) twice in order to fraudulently get goods or services without paying for them. This is possible because digital information can be replicated easily and sent to multiple parties.
Double-spending is prevented in cryptocurrencies by the use of a public ledger that records every transaction made with the coins.
Because each transaction remains permanently in the ledger it becomes nearly impossible to spend a coin that has already been spent and recorded in the ledger.