1998 Ford F350 warning light

Service Engine Soon light is on. Serious?

The light comes on because the truck needs service, but that should be obvious. It might be serious, only reading the service error codes will tell you for sure.

Now if it were flashing, that would mean you should pull over and turn the truck off before really expensive service is required.

This is spelled out in your owners manual. If you lost your, they are available online from Ford.