The database table prefix is somewhat useful for security, as it obscures the table names, making it hard for relatively simple attacks (which rely on the assumption that database tables are using the default names) to succeed.
It's also useful in cases where you want multiple installations of Flarum to use the same database. Each installation would need to have a different prefix, or both installations would end up overwriting each others' data.
You can use any values you like. The general trend seems to be, say, three or four alphanumeric characters followed by an underbar. For my recent test of the second beta, for example, I used "flab2_" as the prefix.
