Originally Posted by Lisanne
What are tension adjustments?
The top thread and the bobbin thread have to work together to form an even stitch with the interlock buried in the fabric. To achieve this, the amount of tension on the top thread and the amount of tension on the bobbin thread need to be appropriate for the type of fabric you are working on and the type of thread you are using.
For example, if you want to machine quilt with invisible nylon thread, you want to reduce your top tension because nylon thread is stretchy. Reducing the amount of tension on it makes sure it doesn't stretch too much while you are stitching and pucker your fabric.
Or, if you are doing decorative work you might want to use a heavier thread on top and reduce the tension on it so it goes through the needle without too much drag.
A heavier thread used in the bobbin may require the bobbin tension to be loosened so the thicker thread can flow easily.
All of this information will be in the manual that comes with your machine. If you are always sewing on the same types of fabrics (cotton) with the same types of thread (regular weight), then you may never need to adjust tension.
Oh, and I wanted to mention that the presser foot lifter (you use your foot to lift or drop the presser foot) is available as an add-on for most machines. I probably wouldn't make that a requirement for the machine you buy if you can add it on later. Not sure that machines in your price range come with it anyway.