In one sentence, describe what the following article is about:

The thread guides are the metal devices that help regulate tension. They are various loops that you run the thread through before looping it into your needle. They keep the thread from getting tangled and distribute the tension evenly from the spool to your fabric. Tension is what keeps your bottom and top stitches in equal tension with one another. In other words, it is what keeps your front and back stitches looking the same. Both the top and bottom tension must work together in order to create consistent stitching. If your top and bottom stitches aren’t even, it may be due to your tension not being right on the top or bottom. Most domestic sewing machines are of the "lockstitch" variety. That means an upper thread and a lower thread "lock" together. When they don’t “lock” together properly, you could have an issue with the tension.
Find the thread guides. Understand tension.