That is a request to fly extra time at straight pay. In the old days, when this was done on paper, there was a white piece of paper where you indicated your desire/willingness to fly extra time at straight pay, and a green piece of paper where you indicated your desire to fly extra time, but not unless it was double pay.

So, when a trip became available, the schedulers would assign it in the following order:

1) guys with a white slip
2) guys on reserve
3) guys with a green slip

More complicated than that, but that's the basics.