To weave (or sew) in the ends is usually the last step of a project. At the very least, one usually needs to weave in the tail end of the yarn from where the first slip knot was made to start the project, and the very last tail end of yarn that remains after binding off. If you used more than one skein and/or more than one color of yarn during the project, there may be more ends that need to be woven into the work.
The following video shows one method for weaving in the ends to a knitted project, which works for either stockinette or garter stitch:
Cushion of Joy 