Some woods, like oak, are easy to stain. The wood cells are very closed and the stain is not soaked into the wood. A soft wood, such as white pine is very porous and will soak up too much of the stain and the grain of the wood will not show through. You need to close up the wood pores in a soft wood which is what a wood conditioner does. The pre-stain conditioner will close the wood pores. These conditioners are like thinned down vanishes or urethanes. They go on clear and only takes 15 minutes to dry.
After the prestain conditioner is dry, go ahead and stain the wood. The wood stain will stay on the surface of the wood and not soak in. The wood grain will show through the stain beautifully. If you did not use the conditioner, the wood stain would be very dark and there would be little or no wood grain showing. The pre-stain wood conditioners are a must on soft woods.
Marie from Oak Harbor, WA, emailed asking about mixing stains. Read her story in the March 23, 2011 Newsletter. This was the I didn't Know That!! article in the October 22, 2013 Newsletter.