The concept of “having some space to move into” is strongest with portraiture, and the fancy word for the technique is to make use of “active space.” The idea is that a viewer will find a photograph more appealing when the subject has room to move into a space in the photograph. It can be a little counter-intuitive - if you’re taking photos, why should you bother thinking about movement? It’s a good point, but we are talking about potential movement in this case. It’s all about perception. When we look at a photograph of a person, our eyes tend to follow the face and where it’s pointed. If the face is positioned at the edge of the frame looking out, then that’s where our eyes go - out of the frame and off to the next photograph. However, if there is active space between the face/eyes and the rest of the photo it feels more natural, meaning the viewer will spend more time looking at the photo. The same goes for any object that moves: cars, planes, animals, etc. The photo will feel more natural and balanced if there is space for the object to move into, rather than it heading out of the frame. So, wherever your subject appears to be headed try to put more space on that side than the other (or the top, or bottom, etc.).