A force field is a volume of space in which objects are subjected to physical force depending on their position in the space. For example, massive objects such as planets create a gravity force field around them that pulls other matter and energy toward them. Similarly, magnets apply force to ferrous metal objects or charged objects around them. Such fields diminish in magnitude as distance to the source increases.
In sci-fi, the field acts like a solid barrier, rather than a volume of effect described above.