ACID (Atomicity, Consistency, Isolation, Durability) is a set of properties of database transactions. In the context of databases, a sequence of operations that satisfies the ACID properties and, thus, can be perceived as single logical operation on the data, is called a transaction.
Atomicity means that any given transaction is atomic - it either apply all its modifications to the database ot none of them.
Consistency guarantees that every transaction brings database from one consistent state to another consistent state.
Isolation is a way in which transactions that happen at the same time are handled by the database. There are different approaches towards that called isolation levels. But, as a general rule of thumb, isolation may be treated as follows - transactions that occur at the same time should be performed one after another/
Durability means that once transaction has finished - all the changes it has made to the database are durable (they will be seen from now on).