Gathering is a sewing term that's fairly common in apparel and craft sewing. It describes when fabric bunches, usually through the use of basting stitches. The purpose of gathering is to create fullness, ruffles or to help in easing pieces together. It's an important step to do correctly because the most important thing is that your gathering is done evenly so it doesn't look unbalanced. But it's actually very easy to do and can create different looks, depending on how much your fabric is gathered. Watch this tutorial to see exactly how it's all done.
Gathering Fabric