Natural numbers are numbers 1,2,3,4,5,..., that are used for counting objects or for indication of serial number of object.
For example, you can count fingers on your hand, count books etc.
Any natural number can be written with the help of digits 0,1,2,3,4,5,6,7,8,9.
Examples of natural numbers are 15, 89, 2, 136 etc.
The smallest natural number is 1. There is no biggest natural number, because whatever natural number you take, you can add 1 to it and resulting number will be bigger.
As example, suppose you think that 22 is the biggest natural number, but by adding 1 to it, you will obtain bigger number 23.