The adjective field in astronomy means disconnected or distant from some objects of interest, but can be used in two situations, one or both which may apply to an object.
Firstly, field is used to describe an object within your field of view (during an observation) as being substantially nearer or further from you than the objects you are observing, i.e., its angular nearness might make it appear to be part of a system (i.e., gravitationally bound), but that is not the case.
Field's other meaning is the same as free-floating, meaning some type of object that is often gravitationally bound to others objects, but in this instance, it is not, e.g., field galaxy.
The phrase field star can have either or both implications. The phrase field planet suggests the same thing as rogue planet (aka free-floating planet, interstellar planet, or substellar object), i.e., not part of a star system. The phrase field object could be used for an object for which it is unspecified whether it is such a field planet or it is a lone brown dwarf, which could be termed a field brown dwarf.
The term field has other astrophysical uses: