The foundation in a yoga pose is what is in contact with the floor. It is the same when you built a house, if the foundation isn’t properly done, the rest will be out of alignment, crooked floors, problems with the walls, than the roof, etc.

In a yoga pose, when the foundation is strong and solid, the rest of your body will be safe and properly aligned. Also the rest of your body will feel lighter, since you need less muscle energy to maintain the pose.

When the foundation isn’t right, for example if in Mountain pose your ankles and arches collapse, you will put at risk what is above that. In this case the knees, the hips and than the spine, will be all affected.

So the first thing to check in a pose is your foundation.