The switch is to reset your mileage trip meter as everyone says. You old it down for about 3 seconds and release when your mileage is reset to zero.
This same switch will also toggle between Miles and Kilometers if you only hold it for 1-2 seconds, if I remember right?
If you hold that button for more than 5 seconds... the passenger seat will drop thru a trap door in the floor. (A little known secret.) Winnebago use to placard this, but in 1995 they remove the placard at the request of the NTSB.
2004 Itasca Horizon ISK-40AD, 350HP Cummins 8.3L
115K miles and the best of 3 Diesels I have owned thus far!