explain what index of means in javascript code example

Example 1: js indexof string

const paragraph = 'The quick brown fox';

console.log(paragraph.indexOf('T'));
>> 0

console.log(paragraph.indexOf('h'));
>> 1

console.log(paragraph.indexOf('Th'));
>> 0

console.log(paragraph.indexOf('he'));
>> 1

console.log(paragraph.indexOf('x'));
>> 18

Example 2: indexof

String1.indexOf(index2);