Idempotency is a property in which if identical request is called multiple times same results will be produced each time and will not have any side-effect.

Non-idempotency Example:

Assume we have a api to deposit a bank account with money

POST /deposit

Call1: POST /deposit 
RequestBody:
{ 
 "amount":1000, 
 "accountNumber": 1234890 
}
Response:
{ 
 "id": 1234,
 "amount":1000, 
 "accountNumber": 1234890,
 "cashBalance": 1000,
}
 
Call2: POST /deposit 
RequestBody: 
{ 
 "amount":1000, 
 "accountNumber": 1234890 
}
Response:
{ 
 "id": 1234,
 "amount":1000, 
 "accountNumber": 1234890,
 "cashBalance": 2000,
}

Even though the above call are identical. Since the above request has a side effect of updating the users cash balance, the amount stored in the persons account will be "2000" instead of "1000".

Idempotency Example:

Assume we have a api to get list of deposits

GET /deposit

Call1: GET /deposit
Response:
[{ 
 "amount":1000, 
 "accountNumber": 1234890 
},
{ 
 "amount":2000, 
 "accountNumber": 2539894 
}]

Call2: GET /deposit
Response:
[{ 
 "amount":1000, 
 "accountNumber": 1234890 
},
{ 
 "amount":2000, 
 "accountNumber": 2539894 
}]

If the above api request is called any number of times, the same response will be returned each time.

Idempotent Http Methods:

GET, PUT, PATCH, DELETE, HEAD, OPTIONS and TRACE http methods are idempotent.

Non-Idempotent Http Methods:

POST method is not idempotent.

What Is Idempotency in Rest Api ?