Variables are containers for memory on a computer, they are associated with a name and data type (such as string, number, etc).
As Chris is me was saying about global variables, every variable has a "scope" which controls which part of your program can access other parts of the program.
A variable in program is the same concept as a variable in math such as "x^2 + 7x + 5 = 0" with x being the variable.
If you're using C++ for FRC programming I'd recommend reading
http://www.cplusplus.com/doc/tutorial/ along with the FIRST manuals of course (control system manuals here,
http://usfirst.org/roboticsprograms/....aspx?id=10934 and C++ manuals here,
http://first.wpi.edu/FRC/frccupdates.html.
LabVIEW also has a great community over at
http://decibel.ni.com/content/community/first