#inject is a method that was introduced to Ruby in a later version than that used by RMVX. I wasn't too sure which version you were using, but I do now. You won't find #inject in VX, though you could add it in yourself. It makes it easier to do accumulated iterations of an enumerable object.
I'm not too great at explaining things sometimes, and #inject happens to be one of those. I know what it does, and how it works, but I get to a point in explaining it where I'm unsure if I'll cause confusion. So instead, I'll point you to here:
http://blog.jayfields.com/2008/03/ruby-inject.html
Because that explains it quite well.
For some examples, which might lead to your making your own understanding:
numbers = [1, 2, 3, 4, 5]
sum = numbers.inject { |result, n| result += n } #gets the sum of all numbers
p sum # 15
sum_with_initial_value = numbers.inject(8) { |result, n| result += n } #sum of all numbers, with initial value of 8.
p sum # 23 (8 + [sum of all numbers] )
product_of_all = numbers.inject { |result, n| result *= n } #product of all numbers
p product_of_all # 120 (1 x 2 x 3 x 4 x 5 = 120)
letters = ['a', 'b', 'c', 'd', 'e']
string = letters.inject { |result, s| result += s }
p string # "abcde"
string_with_spaces = letters.inject("") { |result, s| result += s + " " }
p string_with_spaces = "a b c d e "
The last one looks a bit like that method I showed you.
texts = ["Some line one", "Some line two", "Some line three"]
as_string = texts.inject("") { |r, text| r += text + "\n" }
The result of as_string we would see is:
p as_string # "Some line one\nSome line two\nSome line three\n"
So the effect of all_text is to combine an array of strings into one string, those original strings now separated by a new line escape character.
Hopefully you can get some understanding about it. It is a VX Ace thing, unless you write the method implementation into the Enumerable module in VX. So it won't be too useful for you, but sometimes it's nice to learn about something new.