A user account is a critical part of a VPOP3 installation, but it can be hard to narrow down what it is in non-technical terms.
Essentially a user account is something that someone logs into. A user account will have associated mailboxes, calendars, etc. There is one email Inbox per account. An account can have multiple mail folders (in Webmail or IMAP4) and multiple calendars (in VPOP3 Enterprise).
Commonly a user account is associated with a real-life user.
However, sometimes, especially if using IMAP4 with VPOP3 Enterprise, several users may share an account (eg a 'sales' or 'support' account), or a user may have several accounts (eg to separate different types of usage). Some installations will have a dedicated user account (eg 'postmaster') as a mail server administrator account, but this is not necessary (any user can be designated as an administrator).
A user account is not the same as an email address. An account will have at least one email address, but it can have extra email addresses, and those may be shared with other accounts (eg you can have a single email address whose mail goes into more than one user's mailbox).
User accounts are also used for licensing - licences are purchased for a specific number of 'users'.