This is a very expansive topic. There is a lot to learn when you decide to start practicing Yoga. It is easiest to start with the foundational physical postures and move inward to the foundational breathing exercises and then if the interest serves you, move more inward to the philosophical elements. If this is the path you find yourself on and you are still confused, let me shed some light.
Starting with the physical elements of Yoga. We practice asanas/postures to create an energetic effect on our first Kosha/shell of being, the anamaya kosha. This betters our health and prepares us for more internal layers of our being. Practicing different asanas in a particular order can expedite the process by creating an optimal energetic effect on this physical shell. When you first step on your mat start with the strongest zones of the body (usually the biggest) and move outward to the smaller zones.
Let us use this sequence as an example: Stand at the front of your mat. Inhale arms out to the sides and up. Exhale catch left wrist and side bend to the right. Repeat other side. Inhale hand to the lower back lift the chest. Exhale and backbend with the support of your hands. Inhale arms out to the sides and up. Exhale forward fold and allow yourself to sway a little. Walk out the hips a little. Now, deepen the forward bend by catching the toes with your middle and index fingers or place the palms face up under the fronts of the feet.
After having done this small sequence, can you feel how the body is feeling more capable of doing deeper elements? There has already been warmth and preparation created at the core. Now we can move to the larger legs bones, like the hips and then to the arms, etc. This method of practice keeps you present. It will constantly bring you back to awareness of what feels right in your body vs. what might be merely aesthetic. Another element to consider in your practice is the concept of "pumping" the joints as opposed to merely holding multiple strengthening asanas one after the other. After each bent limb pose include a straight limb pose. This will bring blood and lymphatic fluid in and out of the joint optimizing detoxification and energy movement/production.
Two equations to consider while practicing on the mat: 1) Legs-arms-spine. move from the largest area of the periphery to smallest to pump blood, oxygen and nerve impulses to the center of the body activating the para-sympathetic nervous system. 2) Side extension-backward extension-forward flexion-twisting . It is easiest to imagine this at the spine but the same rules apply to the legs and arms as they are mirror reflections of one another. At the hips/shoulders level you must bend and extend the joint properly before you can twist them as in lotus or virasana. You will always have success in your practice if you prepare yourself in this way.
Now that you understand the why's on a physical level I can give you a teaser into how this starts to move more inward toward your pranamaya kosh/energetic shell . Marmas are energy zones in the body that are larger than meridian points and usually correspond with all joints in the body. The marmas are more physiological than chakras because they can be physically felt. When you have extended postures you expand marma energy. When you fold the body you relax or calm marma energy. When you back bend you provoke and enhance marma energy. When you twist, you unlock marma energy. When you step onto your mat and you begin by setting a goal for your practice, think very seriously about what you are looking to achieve. If we start with a goal in a mind, the visualization of it will keep us present in our practice and draw us back if we begin to wander.
The video above simplifies the concept of side bend, backbend, forward bend without twisting. After properly preparing the core we move into legs where you have forward bent knee posture followed by extension at the hip socket, Archer or side lunge moving the hip outward with bent knee and straightening of the same leg to expand further into the hip socket and behind the knee. Simple series yet profound in its ability to prepare the body for practice.