Definition: Baste when you are sewing, refers to temporarily joining fabric together.
Instructions that request you to baste, want you to temporarily sew with long removable stitches. Basting is the action of sewing a temporary stitch which is easily removed once you do the permanent sewing. It is achieved by making large removable stitches by hand or machine. The more you need to hold something together, the smaller your basting stitches will be but they are always sewn with removal in mind.
Machine basting is also used to ease in and gather fabric.
Basting holds something in place to test the fit of a garment, join two pieces which may be difficult to fit together or to test the look of an alteration before final stitching. It is also used to temporarily close a seam as in inserting a zipper.
The lining fabric is slippery, so I will baste it before I do the final stitching.
Attaching this circle of fabric to that long straight piece of fabric, looks impossible so I will baste it first.