Weightlessness is a condition when object has little or zero acceleration besides the acceleration that initialize its movement. It happens when the object is very far away from massive body (such as planets and stars) which can have gravitational interaction with the object. (The larger the mass of a massive body, the greater the gravitational interaction it can have with the object.) In this case, the object will some how have a very very little gravitational interaction. There is a situation that can give the object zero gravitational interaction. This happens when the object is surrounded by multiple massive objects and their gravitational interactions with the object are cancelled out.          This is exactly what happens to the astronauts. And since human body has been adapted to the Earth gravitation for thousands of years, health problems such as bone loss will happen if staying in the space for too long. Blood circulation will also be hindered in the long run.         Remember Newton's First Law of Motion? He says an object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force, so this is what weightlessness condition can do. Weightlessness condition can be described as a "balanced force" in the second sitution we talk about previously (multiple gravitational interactions with the object are cancelled out).