"1" to "M" or 1 to many is a type of relationship between tables in a relational database. It means that against 1 record in a table, there can be 0 to many rows in the other table. Not that the many or child table doesn't need to have any record against the 1 or parent row, so the many side is optional.
Example can be a Customer and Order relationship. Where Customer is the parent table and Order is the child table. "A customer can have 0 to many orders".
