求解释 Count and Say 这个题目是啥意思

https://leetcode.com/problems/count-and-say/

题目如下：

The count-and-say sequence is the sequence of integers beginning as follows:
1, 11, 21, 1211, 111221, ...

1 is read off as "one 1" or 11.
11 is read off as "two 1s" or 21.
21 is read off as "one 2, then one 1" or 1211.

Given an integer n, generate the nth sequence.

Note: The sequence of integers will be represented as a string.

题目有点绕，看不大明白这个序列怎么来的，有没有地友能帮忙解释一下？Thanks.

1, 11, 21, 1211, 111221, ...
1: 一个1 --> 11
11: 两个1 --> 21
21: 一个2，一个1 --> 1211
1211: 一个1，一个2，两个1 --> 111221
.....