Say them aloud!

$\begin{array} {r|l} \text{In number} & \text{Saying in words} \\ \hline 1 & \blue{11} \text{ (one 1)} \\ \blue{11} & 21 \text{ (two 1)} \\ 21 & 1211 \text{ (one 2, one 1)} \\ 1211 & 111221 \text{ (one 1, one 2, two 1)} \\ 111221 & \boxed{312211} \text{ (three 1, two 2, one 1)} \end{array}$

In the fifth term, there are three ones , two twos and a single, i. e. one one . So the next number is $\boxed {312211}$ .