What is a row split in a MySQL database?

Question

In the MySQL manual:

For a MyISAM table that uses a dynamic row format, updating the row to a longer total length can split the row. if you do this often, it’s very important to use OPTIMIZED TABLES from time to time. See Section 12.4.2.5, “TABLE OPTIMIZATION Syntax”.

Is varchar a dynamic string format? What does "line splitting" mean? How many line breaks affect speed?

How can a line break? If I create my varchars as Varchar (255) and you have 10 of them (Latin1), then even at full length they are under the maximum row size. In this case, is it a line break that I would not see? How to avoid it?