What is a row split in a MySQL database?

From the MySQL manual:

For a MyISAM table that uses dynamic row format, updating a row to a longer total length may split the row. If you do this often, it is very important to use OPTIMIZE TABLE occasionally. See Section 12.4.2.5, “OPTIMIZE TABLE Syntax”.

Is varchar a dynamic row format? What does "s开发者_如何学Goplit the row" mean? How much does a "row split" affect speed?

How is it possible for a row to be split? If I set up my varchars as Varchar(255) and have 10 of them (Latin1) then even at full length they are well under the maximum row size. In this case, is row split something I would not see? How do I avoid it?