What is ACID properties ?

ACID : Atomicity Consistency Isolation Durability 

Atomicity: It is about 'all or nothing'. When the transaction has multiple steps involved then if any one step fail then all the steps should be failed.
It should complete fully only. No partial transaction.

Consistency: It is about the state of the database. Any transaction to the database should bring the database from one valid state to another valid state.
the data in the tables must be consistent as per the rules and syntax.

Isolation: When multiple transaction are running sequentially then completed transaction data must be available to the upcoming transaction.

Durability:  It says that when any transaction commit the data must be available even if the system crash or electric issue or whatever disaster happens. After transaction commit successfully. Data must be available for later on.