Mobility vs. Flexibility
Flexibility is how far a joint can be moved. Mobility is how well you can move it, with your own strength and control. Flexibility is the range; mobility is the range you actually own. For a body that stays capable, mobility is the one worth training.
The difference, simply
Flexibility is passive: gravity, a strap, or someone else moves you into a stretch and the joint gives. Mobility is active: you move the joint through its range under your own control, the muscles switched on the whole way. You can be very flexible and still have poor mobility, plenty of bend, but no command of it.
Why mobility matters more
Real life never asks you to hold a passive stretch. It asks you to reach, twist, climb, and catch yourself. Range you cannot control is range you cannot use, and at the far edge of a stretch, without strength, it can even invite injury. Mobility keeps a joint strong exactly where it is most vulnerable.
How Pilates builds it
This is the quiet genius of Pilates: it moves every joint through its full range with the muscles engaged, never slack, articulating the spine one vertebra at a time, opening the hips with control, guiding the shoulders through their arc. It is not stretching harder. It is learning to move better, and keeping that ability as the years go on.
Train the range you want to keep
A few minutes of slow, controlled movement each day does more than the occasional deep stretch. Take each major joint, spine, hips, shoulders, ankles, gently to its edge and back, on purpose. The range you use is the range you keep.
Don't chase more flexibility. Own the range you already have. Move slowly to the edge of comfort and back, every day, control is what turns bend into strength.