{} is something called a preprocessor meaning that it only affects the outcome of your program during compile time, not run time.
Generally you would include a .i file, not run it but this kind of syntax could be used both for running and including files.
In this perticular case it means that the compiler will insert something where {1} is written. {1} signifies the first argument inserted. You can name arguments as well (see the link below).
A simple include of a file like this:
{filename.i}
In this case it's not very easy to guess whats inserted but basically if you do an include like this:
DEFINE VARIABLE c AS CHARACTER NO-UNDO.
c = "ABC".
{c:\temp\includetest.i c}
The compiler will insert ABC where {1} is written in your include file. That would make no sense in this example of course. And I don't now of anything that really can be written between the parameters and the colon.
Let's say your the filname.i file simply contains.
DISPLAY {1}.
Doing the include like above would then make the compiler replace {1} with "ABC" and thus making the statement look like this:
DISPLAY c.
Running this would display the value of c - "ABC".
You would have to provide the code running or including your file for better understanding of this!
Pages 22 and forward in this document describes this deeper.