A zero confirmation is a transaction that has not been sent to a block.
A normal transaction that receives no confirmations runs the risk of becoming double spending. Double spending occurs when the same bitcoin is used to make multiple transactions. For example, if a bitcoin used in transaction A is immediately used again in transaction B with a larger transaction fee, transaction B will be confirmed faster than A. As a result, the bitcoin intended for A will be flagged as double spent and become permanently unconformable. For this reason, it is wise not to trust coins with a zero confirmation status.