Chart of Accounts question

[hotsteeldesign.]

I'm just getting started with accounting software and am trying out Express. The manual states:

Header account (for subtotals only, no posting)

If the account is a header account it means that nothing can be posted directly to the account but on reports the account prints the subtotals of all accounts below it with the same prefix.

My question is: what is the definition of a prefix?

Thank you,

suzanne