The sewing pattern is the template from which the fabric is cut and then sewn to make your garment.
Many patterns are available commercially, and are quite inexpensive, but sometimes there is nothing available which quite fits the requirements. If that happens there are a couple of ways of tackling it. One is to adapt a pattern which is readily available. This is often a good solution as a lot of work has already been done and you would then only be paying for the time involved in adapting the pattern.
The other way is to draft a pattern from scratch, either hand drawing or using CAD software, bear in mind this is a time consuming operation and will need test of concept as well as fitting so it will need your time and input too.