The soft palate is located behind the hard palate in your mouth (the hard palate being the bony hard area that makes up the roof of your mouth). It's basically a collection of muscles which seperate the nasal passages from the mouth during swallowing. With some practice, you can learn to manually control it, which comes in handy when singing. To get a feel for it, try singing while holding your nose. This will give you a good idea how much air is trying to go out of your nose. Now, while doing this, try to sing so that it sounds the same as it would if you were not holding your nose by forcing all the air out of your mouth. When you succeed, you are forcing your soft palate up, and you should be able to feel it a bit in the back of your mouth.