Atomicity - An atom cannot be subdivided further. So should be the transaction.
Consider a bank transfer account. In that, A transfers 10 to B. So, database'ically',
Operation #1: 10 is subtracted from A.
Operation #2: 10 is added to B.
Atomicity is both these two operations should happen or none should happen. It cannot be only Operation #1 happened or Operation #2 happened. 😀 Both the operations happened is called a Transaction. 😀

Aggregation: There are 7 functions in aggregation in databases. All these can be performed only on multiple rows / data.

AVG()   - Returns the average value
COUNT() - Returns the number of rows
FIRST() - Returns the first value
LAST()  - Returns the last value
MAX()   - Returns the largest value
MIN()   - Returns the smallest value
SUM()   - Returns the sum