We women are designed for birth. Babies are designed for birth. during labor, babies who are in a vertical, curled position descend through the pelvis in a tube-shape. If a baby can't fit because of position, that's a "malposition."
Some providers argue that breech and posteriors can be normal positions. Many midwives will say that breech is not a malposition in and of itself. This is because with physiologically-based birth care, most breeches birth normally.
Figuring there is truth in both sides of a disagreement, I'd say then, that among the so-called malpositions of breech and posterior, there are babies that fit without any trouble in labor. Since not all mothers and babies have the same physiology, we can't expect all breeches and posteriors to have the same ease or same difficulty.
Well fitting anterior, breech or posterior babies are often flexed, their mothers have roomy pelvises. Birth for these breeches and posteriors may go very well if no one interferes by pulling, dosing with epidurals, restricting the birthing woman to back-lying positions, etc.). But there definitely other breech and posteriors that show they are malpositioned by not being able to descend or fit the pelvis spontaneously. Those babies are malpositioned. Since we ALWAYS can't ALWAYS tell which are which before hand, most experts agree that breech and posteriors belong in the "malposition" category. (But not all.) Sometimes a posterior presentation is called a slight malposition.
But some breeches have trouble with the special movements of birth so that they either can't get into the pelvis or they get stuck.
Whether head down or bottom down (breech), many babies will fit well, some will find their way even if they have a bit of trouble fitting due to position and a few won't fit.
What to do?
Mother's may have instinctual movements in labor that correct a malposition but we wouldn't see it if she were in bed being continually monitored or in bed on an epidural which blocks her hormonal communication with her labor. I'm not entirely against epidurals, they are ok when a cesarean is the next step, or to relieve a woman who is truly suffering or can't "let go." But they have consequences that may be potentially long lasting on the hormonal (read brain function) of babies and mothers and relationship building, esp. when Pitocin (syntocin) is added to the mother's body and so the baby's. This needs research, but when Dr. Marshall Klaus is warning the world against these two interventions, we need to listen. They can be the answer to get the baby born, but they are not without effects.
It takes a skilled practitioner to help navigate labor with a slight malposition, if labor is long or borderline difficult. The midwife or doctor must be attentive without scaring and stressing the mother. This is a rare skill, learned by using both sides of the brain, and is one I wish I could boast, but is very much an aspiration.
Find a doula, midwife, doctor, or labor and delivery nurse with training in maternal movement in labor, with labor progress techniques, and who lets you lead your own birth dance.
In your pregnancy, go for balancing your pelvis and whole body to avoid stalls or delays due to long labors. See The 3 Principles in Pregnancy and do what it lists for the 1st Principle, Balance. And here are the pregnancy activities to do first.