All the Format command does is allow you to display variables as part of a string. As an example, if yourgroup is a team of three:
Hint Format ["You have %1 effectives in your group, %2!",Count Units yourgroup,"Triggerhappy"]
would put up a hint saying:
You have 3 effectives in your group, Triggerhappy!
The first element (element 0) of the array is a string with placeholders for the variable values. %1 and %2 are placeholders for the second (element 1) and third (element 2) elements in the array.
That's the gist - but the syntax is definitely not guaranteed this time of night. Don't know what the maximum number of placeholders is, but it shouldn't be a problem most of the time.
There are cleverer uses where you use the result of the Format as a variable name itself but start simply.