Leader Terminology is as follows:
Butt Section- The heaviest portion of the leader that attaches to the end of the fly line.
Mid Section- The portion of the leader forward of the Butt Section.
Tippet- The thinnest portion of a tapered leader.
There are also Bite or Shock Tippets which are used for toothy critters, but don't concern yourself with this tippet for your current application.
X designates the tippet strength. If you want to know the approximate strength of a tippet, subtract the X number from 9 to obtain the strength of the tippet. For example, a 5X leader subtracted from 9 equals 4 pounds test, and a 4X tippet subtracted from 9 equals 5 pounds test. These are all approximate, but pretty darn close.
Leaders come in different lengths (7-1/2', 9', etc.) for different applications. Also, the size and type of fly that you intend to use will have a direct relationship the size leader that you will want to use.
If you purchased your rod as a combo, you should have the correct fly line for your rod. I hope that this helps!