To add to what Kanegasi said...
When you did t[1]=100 you assigned the number 100 to the first index of the table. (You can assign a value to any index, and you don't even have to do it sequentially.) 1 and 100 are then an index, value pair. (Keys are strings, indices are numbers.)
Here is more information on table basics and how to work with tables:
http://lua-users.org/wiki/TablesTutorial
Here is a more in depth tutorial on tables and the table library:
http://lua-users.org/wiki/TableLibraryTutorial