christine, gen2 and chuckfickens have given you the correct answer. If you read out the contents of each line and then convert that description into numbers you get the next line so:
1 = one one = 11
11 = two ones = 21
21 = one two, one one = 1211
1211 = one one, one two, two ones = 111221 etc so
312211 = one three, one one, two twos, two ones = 13112221