Seafoam treatment???

I've read a few times about "seafoam" treatment a few times on here and am curious. In Florida I obviously don't winterize. However, I am out of the country quite a bit, so my boat sits for three months at a time. My engine is an older (1998) Force OB. What is this Seafoam procedure, it seems to be a good way to clean out the engine to make sure it's running as efficiently as possible. The compression is good and she runs well...just want to keep it that way so I can get it sold.