What mean nested??

Answer 1

nested loop: a loop inside a loop
for (int i = 0; i < 10; i++){
for (int j = 0; j < i; j++){

nested if:

if (i < 10){
if (i < 5){

nested array:

int [][] nest = { {1,2,3}, {1,2,3} }

There are many other examples.

Answer 2

In HTML (XML, SGML, etc.) it looks like this:
<Element1>
<Element2>
<Element3>
Some text in nested elements
</Element3>
</Element2>
</Element1>

Answer 3

In some languages it is possible to create nested classes:
https://www.sololearn.com/learn/CSharp/2684/?ref=app