Lower Back muscles is a term that refers to several difference muscles, of varying sizes, of the lower back. Sustained postures can cause tightness in these muscles, e.g. sitting on the edge of your chair, or sitting in a chair with your knees lower than your hips.
The muscles of the lower back include the Latissmus Dorsi, Multifidus, Longissimus and Iliocostalis.
The Latissmus Dorsi (or Lats) is a more superfical muscles, where Multifdus, Longissimus and Iliocostalis are deeper back muscles.
Below is a picture showing the different layers.
Tightness in the lower back muscles can effect the position of the pelvis as well as the curve of the lower back. If these muscles become tight you will typically see an increased lordosis (an increased backwards curve of the lower back) and/or an anterior tilt of the pelvis (the top of the pelvis is rotated forward).
To learn more about lordotic posture click on the link.
Lower back stretches aren't for everyone. They are best suited for someone with tight lower back muscles.
Why is this?
In order to stretch the muscles of the lower back you need to bring the lumber spine into some flexion i.e. bending the lower back forward. If your muscles are not tight you may be stretching the ligaments instead or placing extra stresses on the discs.
This video shows some great techniques for stretching the lower back muscles muscles.