In case you were unaware, a gusset is a triangular or rhomboid piece of fabric sewn into a seam to add breadth or reduce stress from tight-fitting clothing. Ever hear the term "Don't bust a gusset"? Well, now you know what it means!
One of these 'tight' places can be the underarm seams of a bodice. Here's a great example of how gussets help to make a garment more wearable and fitted!