This program models gas chromatography. Gas chromatography is a process of separation of chemicals based on surface interactions. Turtles are molecules which are pumped through the blue "absorber". One kind of molecule is more likely to get "stuck" than the other, slowing it down. This leads to separation of the two types.
For detailed instructions on using the program, take a look at the info window from the Gas Chromatography model.
Click on one of the pictures to see a quicktime movie of the model: