It means that as long as you don't already have the reforms, they will happen as soon as turn 254 hits (or, if some reason, the script isn't running then the reforms will take place when the script is activated again, until turn 660). Change the first number if you want to change the reform date, but you'll have to do it for each settlement.

Eg if you want the Polybian reforms to start at 250BC, you'll have to change the 253 to 87 (250BC is 88 turns after 172BC, but since it's greater than x number of turns rather than greater than or equal to x number of turns you'll have to make it one turn less).

I hope that made sense.