Quick tip for thinking about it: If you think about your hips as a glass of water it might help. Anterior (APT) would pour forward, Post (PPT) would pour behind, and lateral would pour out to a side. So, next time you should be holding your hips square, think about that glass of water
There are numerous people experiencing musculoskeletal problems such as anterior pelvic tilt, posterior pelvic tilt and lateral pelvic tilt. Millions of dollars are spent each year for its treatments because it produces knee and back pain among other medical issues. I'm dealing with this myself, ironically for me I think it's related to how I started to feel workouts a bit more in my back.
Anterior Pelvic Tilt
For anyone who sits most of the day, the anterior pelvic tilt is really a problem. Normally, what happens when you sit a lot is that your butt sticks out while your gut protrudes. Once this happen, you won’t be able to get rid of it by losing fat. People with APT display poor movement patterns in exercises like squat and deadlift. Ironically, I switched to a stand up deck thinking it would “fit everything” but found I still have bad posture. This is due to the overextensions of the lumbar spine and low back dominance.
Exercises: Hip flexor stretches as well as strengthening the posterior pelvic tilt movement while going to the gym for good movement patterns daily all help eliminate APT. I will personally be working on this for a while.
Posterior Pelvic Tilt
Posterior pelvic tilt means your hips are excessively tilted. Aside from back pain you may experience when you have PPT, it may also lead to muscular imbalances if not treated properly. It is actually the opposite of anterior pelvic tilt. It happens when the front of your pelvis rises while the back of your pelvis drops
Exercises: Hamstring and abdominal stretches can strengthen up your hip flexors and help lower back erectors.
Lateral Pelvic Tilt
This musculoskeletal problem is normally associated with scoliosis. It’s a postural problem wherein one side of your pelvis is either tilted to the right or to the left side causing one side of your hip to look like higher than the other. This causes back and/or hip pain as well as postural deviation. For me, my right leg is slightly bowed so my hips have a small one. It's so slight that you can't tell but for some people, you can have a shoulder appear droopy and your leg will look longer than the other.
Exercises: Stretching such as static stretch helps alleviate the pain and help correct the deviation. For me, I have to check my feet before EVERY lift because the position of my feet is important for squaring my hips and it doesn't always feel smooth.
For additional information, you may wanna visit: https://www.youtube.com/watch?v=QSS1D0g6DN4