What is "low muscle tone (hypotonia)"? How does it affect development?

Low muscle tone means the body feels floppy, and movements that lift the arms and legs against gravity are hard. So it can look like there's little movement of the arms and legs while on the back, and holding a posture takes effort. The effects reach beyond movement, spanning areas such as fine motor skills, posture, attention, and sensation (proprioception, the sense of where the body is in space). But this isn't "can't do it"—it's that "building the foundation takes time." Without rushing, and at the child's own pace, things will keep growing.