Fabric is flat. Bodies are round and curvy. Fashion design (and patternmaking) is about taking that flat yardage and molding it around a living, moving, 3-dimensional form. There are several ways to do this.
Darts are one of the big ways to create shape in flat fabric. Darts pinch out the excess fabric and taper out to a point to create more space for the fullest parts of the body like the bust and hips. They can also span between two full areas with a point at both ends and more dart intake in the middle. This is called a fish-eye dart and you’ll often see these run vertically from the bust to hip lines.
Style lines can do the same. They function the same as a dart, but instead of pinching out fabric within one piece, fabric is removed between two pieces. Where the dart would be, there is negative space between the contours of two parts of the garment. In order for these to create the shape needed in a garment, they need to run across the fullest part of the body.
Both darts and style lines remove fabric from the garment in order to create shape, but you can create shape in your designs by leaving the fabric in as well. Pleats, gathers, and ruching are ways to do this. With these techniques, you gather up the excess fabric in one area as part of the design and let it naturally taper out to the fullest part of the body where that additional fabric is needed.
Unless you are designing a very boxy garment or using a super stretchy fabric, you need to employ some of these shaping details in your design in order to fit flat fabric around a very not flat body.