When you first install a package, you are sometimes asked some questions about how you plan to use it; your answers to those questions are then used by the system to do some initial configuration of the package. The command dpkg-reconfigure is used when you want the system to ask you those questions again after installing the package, for example when you change your mind about some of them.
The questions are divided into four priority levels as follows (from man 7 debconf*):
low Very trivial questions that have defaults that will work in the vast majority of cases.
medium Normal questions that have reasonable defaults.
high Questions that don't have a reasonable default.
critical Questions that you really, really need to see (or else).
The --priority flag of dpkg-reconfigure lets you specify the minimal priority level of the questions you will be asked; for example --priority=high says you want to be asked only the questions whose priority level is high or higher, and not be bothered by lower-priority questions. This is what happens automatically when you first install the package.
The source you have been reading, however, considers that, for the package unattended-upgrades, it is important that you answer the low- and medium-priority questions as well. Since you are not asked those questions during installation, it therefore recommends that you run dpkg-reconfigure with --priority=low afterwards.
* This man page is not installed by default; it is available in the package debconf-doc or online.
dpkg-reconfigureis, as its name implies, to configure the software. Presumably, you could also do it by editing config files by hand, butdpkg-reconfigurewill be more user-friendly.dpkg-reconfigure?--priorityin particular that's confusing you? If so, I'll write an answer about that (you may also want to edit the question to make that point clearer).