Formula to this pattern? $1$, $11$, $21$, $1211$, $111221$, $\ldots$

It's known as the look-and-say sequence.

In the first row, there is one 1 (11). In the second, two 1s (21). In the third, there is one 2 then one 1 (1211). In the fourth, there is one 1 then one 2 then two 1s (111221). It continues in this way, so the next row would be 312211.