Agile process model

I’m trying to study for my Computer Science course and I need some help to understand this question.

I need to write a report for the following QS as appear ?

Notes to be considers :

1. Use academic references as required .

2. use not more than 3 resources.

3.Apply APA style.
4. Wiki is not acceptable .

1)introduction for Agile?

2) What is Agile ?

3) When do you use Agile?