a dec is a decrease - that means that you work one of the possible decreases (there are many different ones) to have 1 stitch less (or 2 in double decrease) than before.
Have you noticed the "Free Videos" section up top on this page? There is an awesome listing of decreases with picture, description and video so you can just take a look there.
Does your pattern come awailable anywhere online? If so, please link. If not, maybe you can type the lines in here that you do not understand right / have trouble with. We figure it all out!
Here for example is an instruction and video (my PC here is funky right now so I did not get to start the video)