Even in their resting state, when they appear to do no work, our muscles aren't completely relaxed. In children with low muscle tone, their muscles are more relaxed than normal, so they may appear floppy, or feel soft and a bit squidgy.
This can be a problem when it comes to maintaining a simple posture, such as sitting up straight, simply because they have to work harder at it. My DS2 is demonstrating this quite well at the moment, virtually lying down to eat a sandwich. It also causes him minor problems with walking, since he walks very flat footed with his feet splayed and even though his muscle definition when he's walking is quite good, he gets more physically tired quicker than other 3 year olds typically do.