What do the triangles mean on sewing patterns?

As mentioned they are notches, they are best cut outward and they are used to match up seams, so you too can avoid putting the sleeve in backwards the way us polished pros sometimes do. (haha) Also pay attention to dots, squares, stars and various other markings. The pattern instruction sheet has a little area that describes the markings and what they are used for; it's usually on the first page beside the style illustration or between the illustration and the cutting diagram or just after the cutting diagram. The sheet also has the individual construction steps and that tells you how to apply the markings such as where to match up the notches, dots, stars and whatever. Always cut and sew with the instruction sheet close at hand, follow the instructions on order, and don't skip steps or jump ahead and don't try to sew your project without the help of the instruction sheet.