Database Migration confusion

Hi guys

Currently trying to wrap my head around database migrations, and I guess I just dont get how they are supposed to be used.

I can see that you can create database tables, and also drop them, but what I dont see in the documentation is, say you want to add a field to a table, have you got to create the table again in another file with the changes, or just issue an update?

If someone could explain how they use them it would be appreciated.

Thanks