Hip dysplasia is a congenital or acquired deformation or misalignment of the hip joint. What this means in English is that the hip joint either 1) doesn't line up with the hip socket, or 2) the joint or socket isn't fully formed in some way. This can occur at birth or develop over time. Statistically, girls are diagnosed more frequently with hip dysplasia than boys.
When dysplasia is identified in an infant, it is much easier to treat. Baby hips are made primarily of cartilage, which is more malleable. This is a key time to treat this condition as it can still be reversed.
I don't know anything about the acquired version of dysplasia, but the baby version is nothing to freak out about! One of the primary treatment methods is the Pavlik Harness. It keeps the baby's legs in "froggy" position. Baby can still kick and move, just not straighten their legs.