Where to start?!  For me, the typical use of a Combo box is to provide the
user with a pre-defined list of choices to choose from.  The Combo box has
the ability to allow the user to start typing and automatically start
matching entries in the list.  The predefined list does not have to be static
- it could be the result of a query in Access which has its results based on
earlier choices/decisions made before using the Combo box.  One of the
advantages of using lists/combo boxes with pre-defined lists is consistency
of data entry.  Consider an example where you are requesting someone's
education level and you just give them a Label: "Education Level:" and a text
box to put it into.  They may enter a variety of things such as "GED", "HS",
"1", "4" (is that 1 or 4 years of what? high school, college), "BA",
"Bachelors", etc.  To make any sense out of such a list after a few hundred
or thousand people have taken their best guess at what you wanted for an
answer could be monumental.  If you give them list of choices to choos from
as:
HS/GED
AA
Some College
BS/BA
MS/MA
PhD
then you get consistent data entry to use later for searches, analysis, etc.

Asking where to get information for any product such as Excel or Access is a
pretty generic question.  If you are just beginning, I would quite frankly
recommend one of the "...for Dummies" books.  I've found those to be great
introductions for novice users - they do not assume you understand anything
about the application, so they provide simple definitions of terms you need
to know, and other things related to the subject.  Then they build gradually
on the information presented to more complex tasks.  They are usually pretty
good at introducing you to a very good range of abilities of the application
- so you know what it is good for, or not, later on in life.