What is ArrayList in Java and Why we use it when coding ?

Question

Recently i am perticipating a a trainning session on android .The trine said we will be using arrayList a lot . So just curious about how it really use and why !

Answer 1

ArrayList<Integer> list = new ArrayList<>();
list.add(5);
list.get(0);

Answer 2

An ArrayList is like an array, but you can change it's size.